[dune-pdelab] CMake check for Eigen

[Christoph Grüninger]

Hi PDELab,
find attached a patch to implement a check for Eigen. As there I was not
able to find test code, it is untested.
Additional I moved a function from the PDELab macros to UsePETSc.cmake
where it makes more sense. As I don't use PETSc myself, it is untested, too.

I welcome any constructive criticism and a commit to PDELab master.

Bye
Christoph

-- 
... as Sir Cyril Hinshelwood has observed ...fluid dynamicists
were divided into hydraulic engineers who observed things that
could not be explained and mathematicians who explained things
that could not be observed.                 -- James Lighthill
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