Harmonize Datatype equality checks

[franzpoeschel]

currently

We override operator==(Datatype, Datatype) such that it checks Datatype equality by representation on the platform. However, in some places, our loadChunk / storeChunk API still manually implements this check, with slight implementation differences.

with this pr

This PR merges those differences into the operator==() implementation and uses that consistently.

Also:

• Adds a missing type check for store_chunk / load_chunk in our Python API

• Adds non-template type comparison operators

• Maybe add a further overload to Dataset constructor, to avoid this weirdness:

      e_chargeDensity_x.reset_dataset(DS(np.dtype(np.int_), [10]))