Add tests to make sure we explicity check for `sum == True`

This reverts commit e34b4b73.

Previously, calling an operator would spuriously return a float
when the operator was hermitian, even if the bra/ket vectors
sandwiching the operator were not the same.

Explicitly initialize pointers to NULL, refrain from using
pythonic constructs, e.g. `a or b`, with cython extension types;
this causes cython to produce (correct) code where pointers are
not cast to the correct type.

Viacheslav Ostroukh authored 8 years ago and Christoph Groth committed 8 years ago

    Fixes issue #74

Without this measure, a failed check is no longer detected.  This seems
to be a bug in gitlab CI.