Accept function handles for RHS specification

[Schlevidon]

Changes

Improve interface for RHS input in prepareDatahandleForIntegration.m

• RHS can now be passed as a file (optionally with a preceding path or an extension) or as a function handle.
• Improve help comment to be more user-friendly and descriptive.

Minor refactorings

• Cleanup setUpIntegrator function by using helper functions.
• Move Filippov sliding field initialization to initDatahandleFields.
• Better exception messages and formatting.