modify operator API and condense docstrings.

+ 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.