Unified 25d WFS referencing scheme

[hagenw]

For the start of the discussion have a look at #136.

[fietew]

As far as I could follow the discussion in #136, the integration of the code into the existing framework is still an issue. My suggestion to integrate this unified scheme a little bit more into the existing code would be the following (asterisk for driving_function_mono_ or driving_function_imp_):

• extend *_wfs_<src>, etc. about the optional input parameter dx0
• in *_wfs_<src> for conf.dimension = '2.5D':
  • compute pre-filter
  • compute the directional gradient of the desired sound field (as done in driving_function_mono_unified_25d_wfs)
  • If dx0 is given, ignore conf.driving_function. If no dx0 is given, compute dx0 based on conf.driving_function.
  • compute xPCS and driving function

The drawback would be, that some driving functions, e.g. for point source and focused source, might not fit into this framework.

[hagenw]

I'm also in favour to integrate this in the existing framework.
This pull request will not be merged into master, but we should integrate the changes in another branch I would suggest. I created this pull request mainly in order to easily review the changes proposed by @fs446.
Unfortunately, I'm still a little bit short with time at the moment, but in a few weeks I should also be able to work on this issue. I would propose that we maybe first prepare a new release of the toolbox including all the LSFS changes and after that start integrating this one. Does this sound ok?