Transition FutureWarnings older than X versions to Deprecations?

[effigies]

Continuing on from the ArrayProxy.header deprecation in #521, it'd probably be good to move FutureWarnings from <=2.0 (2.1?) to DeprecationWarnings, more generally.

Not sure how to use the versioned deprecation machinery to deprecate modules (looking at nibabel.minc).

[effigies]

@matthew-brett Good to transition FutureWarnings to DeprecationWarnings?

• minc.py (FutureWarning in release 2.0.0)
• checkwarns.py (FutureWarning in release 2.1.0)
• testing/__init__.py (FutureWarning in release 2.1.0)

Also there are some behavior changes that we've been warning about since v1.1.0:

• trackviz.py (also here, here and here)