Check for empty invariant subspace #50
Description
In issue #48, it occurred that a empty invariant subspace was returned by SLEPc as can be seen here. This might only happen in cases were PETSc/SLEPc compiled with complex support is used (which is actually a misuse, since pyGPCCA should be only used with PETSc/SLEPc compiled with real support), but nonetheless I would like to check for this and handle it.