…ed a call to sum to numpy.sum to gain performance.

…sibility for the user to input a list of word indices as parameter 'context' instead of a list of words.

…ng to trying to make it more compact and versatile.

* Retained the suggested `sum`->`np.sum`
  replacement, which has been tested to
  yield significant runtime gains.
* Dropped unnecessary type/value checks
  that are already run when calling the
  `KeyedVectors.__isin__` dunder method.
* Corrected the docstring to accurately
  document the supported inputs (which
  were already compatible prior to the
  PR this commit is a part of).

…ntains ints.