[ENH] Improved PEP8 Compliance for interfaces/spm

[mgclark]

This PR improves the PEP8 compliance of interfaces/spm and is a partial fix for Issue #597.

The initial PR includes the following:

File            Violations       Resolved      Remain
base.py         13               13 (100%)     0
model.py        115              107 (93%)     8
preprocess.py   151              142 (94%)     9
utils.py        8                8 (100%)      0

Some long docstring lines remain; wrapping them made CircleCI unhappy. Even with those lines remaining, the readability of the code has been dramatically improved.

[coveralls]

Coverage decreased (-0.03%) to 72.231% when pulling 8424b80 on mgclark:pep8/spm into fd4dbde on nipy:master.

[satra]

thanks for this @mgclark