Fix 2D Nx1/1xN inputs to eigen dense vector args

This fixes a bug introduced in b68959e8
when passing in a two-dimensional, but conformable, array as the value
for a compile-time Eigen vector (such as VectorXd or RowVectorXd).  The
commit switched to using numpy to copy into the eigen data, but this
broke the described case because numpy refuses to broadcast a (N,1)
into a (N).

This commit fixes it by squeezing the input array whenever the output
array is 1-dimensional, which will let the problematic case through.
(This shouldn't squeeze inappropriately as dimension compatibility is
already checked for conformability before getting to the copy code).