Fixed details in antsRegistration interface

The generation of output filenames was incomplete for inverse outputs
in both collapsed and not-collapsed modes.

Additionally, collapsed mode for only-linear registrations was broken
as the warp file was not generated (but checked for existence)
Minor fix ensuring good values in output

invert_flags in collapse mode got unexpected values (not boolean)
for nonlinear transform
set default option
trailing space is probably making travis fail in #798
Fixed bug with ants initialization

On one hand, invert_initial_moving_transform was used and needed to
be defined. On the other, as there is a xorg spec with initial_moving_transform_com,
it was still failing.
PEP8 formatting
Merge branch 'master' into bug/FixAntsInitialization
Merge remote-tracking branch 'origin/pr/798'