Add some accessors for MATModel

[stelmo]

Add some small accessors to MATModel so that the consensus yeast model can at least be imported without error. The model uses too many non-standard fields to make the import process totally smooth...

[cylon-x]

🟢  Commit 31a4490 is formatted properly.

[exaexa]

minor non-critical improvements possible but it looks okay to me...

maybe we could just throw the yeastGEM model to the tested ones?

[codecov]

Codecov Report

Merging #602 (c5ccab9) into develop (d336a7d) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #602      +/-   ##
===========================================
+ Coverage    90.29%   90.35%   +0.05%     
===========================================
  Files           69       69              
  Lines         1721     1731      +10     
===========================================
+ Hits          1554     1564      +10     
  Misses         167      167              

Impacted Files	Coverage Δ
src/base/types/MATModel.jl	91.80% <100.00%> (+0.89%)	⬆️
src/base/utils/guesskey.jl	75.00% <100.00%> (+12.50%)	⬆️

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[cylon-x]

🟥  Commit 536e679 requires formatting!

Required formatting changes summary:

 src/base/constants.jl      |  5 ++---
 src/base/types/MATModel.jl | 12 ++++++++----
 2 files changed, 10 insertions(+), 7 deletions(-)

[cylon-x]

🟢  Commit c5ccab9 is formatted properly.

[exaexa]

lgtm&fftm

also I realized that reaction_name and metabolite_name gets kinda auto-tested when converting the models around to JSON/SBML ones, which is great.