Extend Data.__contains__ to evaluate subarray presence

[sadielbartholomew]

Presently Data.__contains__ only tests whether scalars are contained within a given data array, but the method could be extended to become more general, namely to check for the membership of sub-arrays, with agreed behaviour for selected cases. Assuming, and when, Python array standards such as numpy support this, we agreed to re-consider the scope of __contains__ in this manner.

See #320 (comment) for context and brief discussion.