Prevent track from birth component w/o detection

[ekhunter123]

Problem: In Gaussian-mixture based trackers, tracks can currently be made from the birth component without a detection.

The GaussianMixtureHypothesiser creates prediction-detection pairs as hypotheses. Each prediction is paired with each TrueDetection and also with a MissedDetection. If the prediction is from the birth component, the hypothesis will get assigned a new tag. Otherwise, the hypothesis will carry the tag from the predicted component. But if the hypothesis is from the birth component with a MissedDetection, then it should not get a new tag. It should keep the "birth" tag because it does not correspond to any real data. This problem is realized later, when the PointProcessMultiTargetTracker checks the tag of each component. If a component does not have the 'birth' tag, then it will try to make a track or add to an existing one. For this if-statement to be effective, we need the birth component to keep its birth tag.

To fix this, I added a new conditional when assigning tags to hypotheses.

---

Also, pointprocess.py use '0' as the tag of the birth component, , while these other files use 'birth':

Stone-Soup/stonesoup/tracker/tests/test_pointprocess.py

Line 26 in d38a196

tag="birth",

Stone-Soup/stonesoup/hypothesiser/gaussianmixture.py

Line 64 in d38a196

tag=component.tag if component.tag != "birth"

I changed pointprocess.py to also use 'birth'.

It may be good to explicitly say that the birth component must have tag 'birth' in the docstring of the birth_component, and/or to check for it in the constructor and raise an error otherwise. Thoughts?

[codecov]

Codecov Report

Merging #628 (ef1cb4d) into main (d38a196) will increase coverage by 0.01%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##             main     #628      +/-   ##
==========================================
+ Coverage   94.22%   94.24%   +0.01%     
==========================================
  Files         158      158              
  Lines        7832     7840       +8     
  Branches     1506     1508       +2     
==========================================
+ Hits         7380     7389       +9     
  Misses        343      343              
+ Partials      109      108       -1     

Flag	Coverage Δ
integration	67.81% <88.88%> (+0.02%)	⬆️
unittests	92.00% <88.88%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
stonesoup/tracker/pointprocess.py	93.54% <0.00%> (ø)
stonesoup/hypothesiser/gaussianmixture.py	92.50% <100.00%> (+1.32%)	⬆️
stonesoup/types/state.py	99.59% <100.00%> (+<0.01%)	⬆️
stonesoup/updater/pointprocess.py	98.57% <0.00%> (+1.42%)	⬆️

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[sdhiscocks]

It may be good to explicitly say that the birth component must have tag 'birth' in the docstring of the birth_component, and/or to check for it in the constructor and raise an error otherwise. Thoughts?

Sounds good. I think we should probably make an explicit definition for it to avoid issues with assuming what the value is.

diff --git a/stonesoup/types/state.py b/stonesoup/types/state.py
index b86c957f..968bbd18 100644
--- a/stonesoup/types/state.py
+++ b/stonesoup/types/state.py
@@ -420,6 +420,8 @@ class TaggedWeightedGaussianState(WeightedGaussianState):
     """
     tag: str = Property(default=None, doc="Unique tag of the Gaussian State.")
 
+    BIRTH = 'birth'
+
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
         if self.tag is None:

such you can check with for example if component.tag == component.BIRTH:

[ekhunter123]

Sounds good. I think we should probably make an explicit definition for it to avoid issues with assuming what the value is.

I like that! I made a new commit where I added that BIRTH constant and changed all the files to use it. Did I do that correctly?

[jwragg-dstl]

All reviewed. Nothing to add from me. Looks to retain the 'Birth Tag' correctly. Thank you very much.

[jwragg-dstl]

Just adding the approval. Thank you.