Spectral derivatives of r(theta,phi), gauss-legendre quadrature and various fixes

See merge request !3

Add computation of derivatives of r(theta,phi) from spectral expansion...

See merge request !2

Added Kerr-Schild metric and extr.curvatuve in spherical polar coordinates for any BH spin (https://arxiv.org/abs/1704.00599). To be further tested.

Implemented correction for interpolation of data with symmetry. Not yet working; need to re-check parity factors and logic/index masks.

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.