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.

It has the same problems as setup_requires.

Packages that are mentionned there are installed in a way that bypasses
pip:
https://pip.pypa.io/en/stable/user_guide/?highlight=setup_requires#installation-bundles

These prefixes were visible, for example when invoking
"./setup.py --help build".

+ References to numpy arrays are removed from docstrings, instead
  general sequences are accepted

+ `__call__` no longer returns `where`

+ a `sum` attribute was added (and the corresponding arg in __init__. If
  sum==True then __call__ evaluates ∑_{iαβ} φ*_α Q_{iαβ} ψ_β. If
  sum==False then __call__ evaluates ∑_{αβ} φ*_α Q_{iαβ} ψ_β (i.e. no sum
  over the site/hopping index i.