Remove Eigen from public interface

[garth-wells]

Having Eigen in the public interface can cause all sorts of problems around alignment. Eigen has some compiler-time settings that set the alignment, but these are not necessarily common across libraries that are linked. Aligned Eigen objects use different 'free' functions, and mixing alignments leads to segfaults.

Since two upstream compiled dependencies may use different alignments the only robust solution appears to be removing Eigen from the public interface.