feat: check SBML validity

[BenjaSanchez]

Main improvements in this PR:

Following up on the discussion in #125, saveYeastModel.m now errors if the saved model is not an SBML valid structure.

I hereby confirm that I have:

• Tested my code with all requirements for running the model
• Selected devel as a target branch (top left drop-down menu)
• If needed, asked first in the Gitter chat room about this PR

[edkerk]

Would be informative to print any occuring errors: include errors in the error() message

[BenjaSanchez]

@edkerk TranslateSBML already prints those errors when called. For instance, if 2 gene labels are removed, the following is printed by saveYeastModel:

The model contains 2 errors.

************************************************************
Line ErrorId Severity Message

122963: (2021203)  Error A <GeneProduct> object must have the required attributes 'fbc:id' and 'fbc:label' may have the optional attributes 'fbc:name' and 'fbc:associatedSpecies'. No other attributes from the SBML Level 3 Flux Balance Constraints namespace are permitted on a <GeneProduct> object. 
Reference: L3V1 Fbc V2 Section 3.5
 Fbc attribute 'label' is missing from 'geneProduct' object.


122964: (2021203)  Error A <GeneProduct> object must have the required attributes 'fbc:id' and 'fbc:label' may have the optional attributes 'fbc:name' and 'fbc:associatedSpecies'. No other attributes from the SBML Level 3 Flux Balance Constraints namespace are permitted on a <GeneProduct> object. 
Reference: L3V1 Fbc V2 Section 3.5
 Fbc attribute 'label' is missing from 'geneProduct' object.


Error encountered during read.

Error using saveYeastModel (line 28)
Model should be a valid SBML structure. Please fix all errors before saving.