Add validation for visualization files

[dweindl]

Some basic validation of visualization files. Closes #8, closes #1.

[codecov-commenter]

Codecov Report

Merging #184 (a90d309) into develop (a0817db) will decrease coverage by 0.25%.
The diff coverage is 54.28%.

@@             Coverage Diff             @@
##           develop     #184      +/-   ##
===========================================
- Coverage    78.55%   78.29%   -0.26%     
===========================================
  Files           29       29              
  Lines         2742     2479     -263     
  Branches       659      575      -84     
===========================================
- Hits          2154     1941     -213     
+ Misses         427      395      -32     
+ Partials       161      143      -18     

Impacted Files	Coverage Δ
petab/petablint.py	0.00% <0.00%> (ø)
petab/visualize/lint.py	55.38% <55.38%> (ø)
petab/C.py	100.00% <100.00%> (ø)
petab/lint.py

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