Fix mapping in TPR tree causing error when mapping is tuple

[sdhiscocks]

This was caused by tuple being assumed to be 2d-slice.

[codecov]

Codecov Report

Base: 94.66% // Head: 94.66% // No change to project coverage 👍

Coverage data is based on head (fb20bc5) compared to base (2b69b97).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #720   +/-   ##
=======================================
  Coverage   94.66%   94.66%           
=======================================
  Files         171      171           
  Lines        8773     8773           
  Branches     1707     1707           
=======================================
  Hits         8305     8305           
  Misses        342      342           
  Partials      126      126           

Flag	Coverage Δ
integration	70.02% <ø> (ø)
unittests	90.75% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
stonesoup/dataassociator/tree.py	97.29% <ø> (ø)

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