Incorporated initialization of 'rh' in the RadiusFromSphericalHarmonics()...

Incorporated initialization of 'rh' in the RadiusFromSphericalHarmonics() method. Separated routines for Athena data into 'data_athena.py'. Collected the tensor indexes and labels in a single place and in 'data.py'. Added a 'data_options' argument to InputData() and run() methods to pass variable options (**kwargs) to various data readers. Started some code to implement interpolation from data with basic symmetries.