Correct options on Eigen::MappedSparseMatrix & adding MSVC C4127 suppression around Eigen includes. (#3352)

* Adding MSVC C4127 suppression around Eigen includes.

* For MSVC 2015 only: also adding the C4127 suppression to test_eigen.cpp

* Copying original change from PR #3343, with extra line breaks to not run past 99 columns (our desired but currently not enforced limit).