Core classes equality test

[pp-mo]

core classes support __eq__
related to #70
Based on dataset-difference or variable-difference
(with obvious possible performance costs, to be warned !)

Formerly included in #79, let's split it out

[pp-mo]

Closed (or partially) by #107

Possibly to re-consider the NcData and NcVariable comparison, but for now excluded + documented as such.