performance: Edge initialization

[nkemnitz]

Proposed changes:

• only set self._affinities and self._areas if they are not provided
• np.array( will only create copies if necessary (e.g. because the data type is not what it should be)

The second one might be a bit risky? Depending on what the original, intended behavior was: the default behavior for np.array in __init__ is to always create a copy, but the current setters are only updating the reference

[akhileshh]

Wasn't aware of the numpy.array's copy argument, thanks!

[nkemnitz]

Oh, there is also np.asarray which does the same and avoids copies by default