RF: Implemented the beginning of a base-class.

Also: converted dti.design_matrix to use gtab as input. This required changes
upstream in predict/adc/tests.
DOC: added a docstring.
WIP: Trying to inherit more models from ReconstModel/ReconstFit
BF: Initialize the base fit object with self.
Inherit OdfModel from ReconstModel, OdfFit from ReconstFit.
Relegate checking for nans to the GradientTable object.
RF - pass dummy gradient table to OdfModel/OdfFit tests
Merge pull request #8 from MrBago/model_base

RF - pass dummy gradient table to OdfModel/OdfFit tests
Avoid possibly changing the user's input array.
DOC: duh.
RF: Clean up some commented code that doesn't do what it's supposed to.
Merge pull request #247 from arokem/model_base

Model base