[eigen] mpreal + MatrixFunctions

[何震邦 <jdh8@xxxxxxxxxxxxxx>]

Hi, all:
I'd like to add mpreal compatibility into MatrixFunctions (i.e. when 
MatrixFunctions and MPRealSupport are both included, then it activates 
matrix functions for mpfr::mpreal), but I can't figure out a way to 
accomplish it, because it's impossible to just add a 
computeUV(mpfr::mpreal) since mpfr::mpreal may be not declared. On the 
other hand, if I code computeUV(mpfr::mpreal) at the other header, it's 
out of the question to add it into class MatrixExponential. Does anyone 
has suggestions?

Sincerely,
Chen-Pang He