#59 missing PVM still raises warning

[Raik Gruenberg]

Since version 2.4 Biskit was supposed not to raise any more warning if PVM is not installed. Instead, exceptions should be thrown only when PVM is actually supposed to be used.

Instead we still get the old and tiring:
Warning (ignored): Could not import PVM (Parallel Virtual Machine) modules. Please check that PVM and pypvm are installed!
Parallelisation is not available.