RAT seems to crash with multiple custom models defined... #422

Using the custom layers example (customLayersDSPC_script), if we add a second custom model (even if unused), RAT crashes on entering the mex.

If a second model is added within the script as follows:

`%%
% Add the custom file to the project....

problem.addCustomFile('DSPC Model','customBilayerDSPC.m','matlab',pwd);
problem.addCustomFile('DSPC Model 2','customBilayerDSPC.m','matlab',pwd);`

..when we try to run it, it fails with an error:

`>> [problem,results] = RAT(problem,controls);
Starting RAT ______________________________________________________________________________________________

Incorrect size for expression 'problemStruct.customFiles': expected [1x:10000] but found [2x1].

Error in RATMain_mex

Error in [RAT](matlab:matlab.lang.internal.introspective.errorDocCallback('RAT', '/Users/arwel/Documents/coding/RAT_fork/API/RAT.m', 75)) ([line 75](matlab: opentoline('/Users/arwel/Documents/coding/RAT_fork/API/RAT.m',75,0)))
[problemStruct,result,bayesResults] = RATMain_mex(problemStruct,controls);
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^`