fix the conversion of python data types to aron data type strings

Aron ND Arrays have a string property to describe the data type of their value. However, it is currently set incorrectly when creating ARON ND arrays from python numpy arrays. This is fixed by introducing a lookup table for python dtypes to aron type names.

As the existing mapping from aron types to python types is not bijective, the lookup table represents only one of a few options.

@peller: Thanks for helping to debug the problem!