Fix sample_condition so events with onset=0 are not ignored
Started to refactor temporal subsampling to allow negative and numm onsets
BF: Fixed the tests to take into account min_onset + made min_onset relative to frmateimes[0]
ENH: Removed the constraint on frametimes and the corresponding test
ENH: Took this opportunity to clean test_hemodynamic_models
ENH: idem with test_dmtx
ENH: took this opportunity to remove an unsued variable n_event in Paradigm class
Added warnings so that events are not silently ignored
BF: added two tests... and corrected some bugs in the warnings
ENH: expose the min_onset parameter
ENH: addressed the PR comments, pep8
BF: underscore some function names that are not meant to be in the public API
Merge pull request #281 from bthirion/fix/sample_condition

MRG: Fix/sample condition.

This one is based on Michael's work to handle cleanly the case where the onset=0 is used.

The main difference now is that we removed all constraints on the frametimes or the condition onset.

We have introduced a min_onset variable, defaulting at -20s, to start sampling possible response.

I have taken this opportunity to improve slightly the test syntax.