UMFPack Memory Limitations

Currently, UMFPack throws an out of memory exception when the memory consumption becomes larger than 2GB.

Inspired by this thread (https://forum.kde.org/viewtopic.php?f=74&t=123386), I was able to circumvent the problem (see attached umfpack_diff.txt).

I'm not sure if using the signatures for long int should be the default or if there should be some kind of specialization.