Feature/use umfpack dl func versions

Fixes #33 (closed)

• Introduces a template parameter Index for ColCompMatrix
• Uses the *_dl_*-functions of UMFPack to go around the memory limit of the *_di_*-functions

mentioned in issue #33 (closed)

Superseded by !301 (merged).

closed

mentioned in merge request !316 (merged)

mentioned in commit d4ddd882