Matroid serialisation and matroid encodings

[Sequenzer]

This adds the ability to save matroids using the mrdi file format. Currently this only allows you to save matroids whose groundset is of type Vector{Int}, arbitrary groundsets are not possible with this implementation. Maybe something for @antonydellavecchia to look into.

This also adds a functions, matroid_hex, which is a way of encoding a matroid into a string using revlex_bases_encoding.

matroid_hex is an encoding used in the paper Quantum automorphisms of matroids.

The encoding style also gets a function to retrieve the matroid from the encoded string matroid_from_matroid_hex. Suggestions for the names of the functions are welcome :)

This is part of the step-by-step merge of the QuantumAutomorphismGroups.jl package.

@danteluber Please Look at this!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.01%. Comparing base (686a746) to head (d37a44c).
Report is 14 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3886      +/-   ##
==========================================
+ Coverage   83.99%   84.01%   +0.02%     
==========================================
  Files         591      592       +1     
  Lines       81386    81750     +364     
==========================================
+ Hits        68359    68683     +324     
- Misses      13027    13067      +40     

Files	Coverage Δ
src/Combinatorics/Matroids/properties.jl	99.17% <100.00%> (+0.06%)	⬆️
src/Serialization/Combinatorics.jl	86.48% <100.00%> (+6.48%)	⬆️
src/Serialization/containers.jl	87.74% <100.00%> (+0.08%)	⬆️

... and 16 files with indirect coverage changes

[thofma]

I am not sure if the "draft" status means that more comments are welcome, but here is a minor comment. The function signatures are sometimes annotated with an explicit convert on the return type, e.g.,

function matroid_hex(M::Matroid)::String

We usually don't do this and I think in none of the cases here it is necessary.