Drop RcppEigen dependency, implement basic Eigen -> C++ interop

[andrjohns]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

We currently have RcppEigen as a suggests for use with the model methods and function exports. However, given that we already have the Eigen headers in the cmdstan install, we only need the methods for converting between R and C++ Eigen types.

Since only a reduced subset of Eigen types are exposed via Stan (Eigen::Matrix<>), this is fairly simple to to handle ourselves.

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Andrew Johnson

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d444e7f) 88.37% compared to head (6f775f9) 88.37%.

❗ Current head 6f775f9 differs from pull request most recent head b33dd02. Consider uploading reports for the commit b33dd02 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #899      +/-   ##
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- Coverage   88.37%   88.37%   -0.01%     
==========================================
  Files          12       12              
  Lines        4535     4532       -3     
==========================================
- Hits         4008     4005       -3     
  Misses        527      527              

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[jgabry]

Looks good to me! Always nice to drop another dependency. If this is fully ready go ahead and merge.