Augmented-data projection (core part)

[fweber144]

This PR adds support for the augmented-data projection to solve #70. For now, I consider it as experimental because

1. I still need to add more thorough unit tests for the newly supported models (families brms::cumulative() and brms::categorical(), both with and without multilevel terms, as well as rstanarm::stan_polr() fits which cannot feature multilevel terms);
2. the augmented-data projection needs to be investigated more thoroughly from a theoretical point of view.

Because of these unfinished aspects, I put "core part" in the PR title. Despite this experimental state, I thought it would not harm to create this PR now so it can already be reviewed and so users can test the augmented-data projection before it is released on CRAN.

Copyright/license:

Similarly to PR #287, the code for the new repair_re() methods introduced by this PR is inspired by (i.e., it uses code snippets from) lme4:::predict.merMod(). I included corresponding comments in the code right before the definitions of these methods and updated the LICENSE file correspondingly. However, this PR also introduces the internal mkNewReTrms_man() function which is basically a copy of lme4::mkNewReTrms(). I have added a corresponding entry in the LICENSE file and (since we now basically have a copy of lme4 code in projpred) also added the lme4 authors as copyright holders to the Authors@R field in the DESCRIPTION file. So the question to @bbolker (and all other lme4 authors) is if you are ok with being listed as copyright holders in the Authors@R field and if that entry is ok this way (see these lines).

EDIT: Point 1 above (thorough unit tests) has been accomplished now (current status: commit 5fe5839).

[bbolker]

Sorry, this slipped through the cracks. I am fine with this, and I'm 99% sure that Martin Maechler and Doug Bates are as well (they're both staunch free-software people, and this is all GPL-3, so really you can use this any way you want ...) I will pass this along to them, but I think you're fine.

[fweber144]

Read through all docs and did not find any errors or strane things

Thanks