Improve parameter acceptance and validation in prepareDatahandleForIntegration

[Schlevidon]

Summary

There are a few improvements in the interface of prepareDatahandleForIntegration which we would like to implement.

• Rename the parameter that accepts a numerical integrator from solver to integrator.

Caution

Don't forget to also adjust the documentation, in particular the readme and github-pages website! There may also be other pieces of documentation aside from the ones mentioned here, so try to be as thorough as possible to ensure consistency across the board.

• Make the integrator parameter accept both function handles and char arrays as inputs.
• Add user warnings for unrecognized or unused parameters.

Proposed solutions

• Check if integrator parameter is a function handle using isa(x, 'function_handle' ). If so, then convert it to a char array using func2str(x).
• Use optionlists for parameter validation. This functionality is bundled with IFDIFF and can be found in the Tools folder of the project.
  • We do not use MATLABs native argument validation because this feature was only introduced recently (2019b) and we want to stay backwards compatible (to a reasonable degree, so stick to around MATLAB2014 if possible).

[Schlevidon]

Implemented in #20