[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. 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.