separate data validation and serialization in serialize.py

[sbillinge]

the serialize and deserialize functions contain extensive code for validating that the data is of the right form. This functionaliity would useful as a separate function in a module called diffpy.utils.validators.py where I could easily check that all my multi-dimensional data is the right shape and so on, regardless of whether I got it from loadData or not.

This issue is to pull the validation code out of serialize into validate functions that are available separaterly and then change the serialize code so it calls the validate functions to give the same functionality.