Only require MultiObjectiveSettings if needed

[volkm]

Running storm-pars lead to an error Cannot retrieve unknown module 'multiobjective'.. This is fixed now by only requiring the setting if needed. There might be a more elegant solution though.

Was introduced with #805

[sjunges]

I don't understand how this fixes the problem. Wouldnt the fix be that storm-pars needs to initialize multi-objective settings?

[volkm]

Let's call it a workaround ;). The proper fix would indeed be to initialize the multi-objective setting in storm-pars as well. However, this settings module should not be relevant for storm-pars because we cannot do multi-objective queries on parametric models (as far as I know). So adding this could introduce some confusion.

Maybe the best way for now would be to indeed add the setting to storm-pars.
Then long-term, we could try to separate some functionality in model-handling.h which can be used in general (storm, storm-pars, storm-pomdp) and specific functions which are only relevant for some binaries (such as functions for multi-objective, counter-examples, game-based abstraction, etc.)

[tquatmann]

Since #805 both options --io:propsasmulti and --multiobjective:lex could eventually be abolished since we now have first-class support of multi-objective and lexicographic properties in PRISM- and JANI-style specifications.

For now, this PR works fine as a workaround and we don't lose anything by merging it.

[sjunges]

Given the discusson above, lets merge this but maybe track this in an issue