Specify default directly in sensmodel arg

[Bisaloo]

I like this approach because:

• it simplifies the code by relying on R idioms
• it makes clear what the defaults are directly in the documentation

If you like this, I can review all functions to see where this can be implemented.

[thomased]

Yeah that's a great idea — the more explicit the defaults, the better.

[Bisaloo]

@thomased, thoughts on https://github.com/rmaia/pavo/pull/227/commits/852814da090d2e5d5013f53ad10622e16342a3ff? Do you find it clearer or no?