Removed all regions conversion functions

Every function for converting a region from a storage format (list,
array, labels) to another has been removed. Tests using these functions
have been adapted.
Removed (un)apply_regions

These two functions have been superseded by (signals|img)_to_(labels|maps)
Adapted plot_regions_covariance.py

plot_regions_covariance.py was making use of the old region API.
The behaviour has been slightly changed: the confounds are now computed
on the whole image, not just on the mask.
Alternate version of plot_regions_covariance.py

Cleaning of signals is performed after region signals extraction.
Result is mostly unchanged compared to previous state.