Update evalModelOptions interface and implementation

Update code so that evalModelOptions completely hides the implementation of the options, similarly to the built-in optimoptions. In particular, change the following common pattern:

evalModelOPTIONS.localModifiableParameterIdxs(i) = [];
evalModelOptions(evalModelOPTIONS);

to something more like

evalModelOptions(evalModelOPTIONS,'LocalNonmodifiableParameterIdx',i);

Moreover, store whole paramSpecs, instead of copying over ony selected fields/properties. Consequently, implement EvalModelOptions as a class, similarly to optim.options.Fmincon etc. built-in implementations, and change current fields like modifiableParameterIdxs into no-arg methods.