Merge branch 'feature/fix-amg-umfpack-when-using-float' into 'master'

[bugfix] This fixes an issue when UMFPack is found and AMG with field type float is used.

The selection of UMFPack as a direct solver does not only depend on UMFPack being found, but also whether the field type is double or complex<double>. Otherwise UMFPack cannot be used as a coarse solver in AMG. This PR fixes this issue.

@markus.blatt: Please take a look at this. 
@smuething: Please take a look at this. 

See merge request !51