Code review items for dipy.reconst.shore

[matthew-brett]

From #363 - useful code review from Ariel:

• Can the docstring code here: https://github.com/nipy/dipy/blob/8abe71f9e2746c7107e0ff5cccb068adb9f00947/dipy/reconst/shore.py#L145 be made into a doctest?

• This error: https://github.com/nipy/dipy/blob/8abe71f9e2746c7107e0ff5cccb068adb9f00947/dipy/reconst/shore.py#L169

• Calculating tau when it's not defined: https://github.com/nipy/dipy/blob/8abe71f9e2746c7107e0ff5cccb068adb9f00947/dipy/reconst/shore.py#L178

• This error: https://github.com/nipy/dipy/blob/8abe71f9e2746c7107e0ff5cccb068adb9f00947/dipy/reconst/shore.py#L180

• This one is tricky. What happens if no solution if found: https://github.com/nipy/dipy/blob/8abe71f9e2746c7107e0ff5cccb068adb9f00947/dipy/reconst/shore.py#L267

• This caching trick (the documentation there could also be better): https://github.com/nipy/dipy/blob/8abe71f9e2746c7107e0ff5cccb068adb9f00947/dipy/reconst/shore.py#L339

• This method: https://github.com/nipy/dipy/blob/8abe71f9e2746c7107e0ff5cccb068adb9f00947/dipy/reconst/shore.py#L461

• This error: https://github.com/nipy/dipy/blob/8abe71f9e2746c7107e0ff5cccb068adb9f00947/dipy/reconst/shore.py#L741

• This error: https://github.com/nipy/dipy/blob/8abe71f9e2746c7107e0ff5cccb068adb9f00947/dipy/reconst/shore.py#L778

I admit that most of these are rather trivial. Error handling is a no-brainer and I don't mean to pick on anyone. Just thought I would make a list of these things, so that it's easier to go back and improve testing, when we get a chance.

[kushaangupta]

1. Converting the example code into doctest is giving an error due to the assumption of the code that data exists in file ISBI_testing_2shells_table.
   https://github.com/nipy/dipy/blob/8abe71f9e2746c7107e0ff5cccb068adb9f00947/dipy/reconst/shore.py#L145