Added an eigenvalue solver to implement power method

Iterative method, repeatedly applying the RHS function to
a solution. The result is the largest eigenvalue and
corresponding eigenvector.
Merging power eigenvalue solver

Simple method to find the largest eigenvalue: Iterate by
feeding the output into the input. Outputs the eigenvalue
and eigenvector.

Conflicts:
	include/bout/solver.hxx
	src/solver/impls/makefile
	src/solver/solverfactory.cxx