Feature/issue 163 naming consistency

[venpopov]

Summary

This PR changes the easier things from #163:

• fit_model() -> bmm()
  • deprecated but can still use fit_model() with a warning message
• dIMM and friends -> dimm
  • hard deprecation
• setsize argument -> set_size
  • deprecated but still usable with a warning
• OberauerLin_2017 -> oberauer_lin_2017
• ZhangLuck_2008 -> zhang_luck_2008
• resp_err -> resp_error
• deprecate parallel option
  • still usable with a warning, but should just use cores = N where N is the number of chains to run in parallel, just like in brms
• bmmmodel class -> bmmodel

Since restructuring the models to use a version argument would require a bigger change and testing, I decided to make this into a separate PR.

Tests

[X] Confirm that all tests passed
[X] Confirm that devtools::check() produces no errors

Release notes

[GidonFrischkorn]

Phew... this was tracking a lot of find & replace details. I tested the changes and added warnings you implemented on my local testing codes and everything ran as expected. This has reduced inconsistencies throughout the package a lot, but it is likely we will still find some every now and then.