Fixed complex linear solve (and some extra)

[mipals]

Hey again Boris, I played a little around with your code.

Here are some very simple fixed so that:

1. You can solve linear systems with a Vector as a right-hand side
2. Solve complex linear systems.

I still believe there are some (very simple) improvements to be made (however, I currently do no have time to fix this):

• Automatically promote to a common datatypes (so that you can multiply a complex HssMatrix with a real Vector).
• Returning a Vector instead of a Matrix when solving a linear system with a Vector as its rhs.

Cheers, Mikkel