change sequence / segment attribute names 'onset_Hz' and 'offset_Hz'

[NickleDave]

@yardencsGitHub points out 'Hz' is confusing since it is a frequency -- I used it because I wanted something quick to type (and I'm a bad physicist who abuses units of measure)

Maybe 'onset_sample_ind' and 'offset_sample_ind'? 'onset_index' and 'offset_index'?
This is one of those cases where there's no good short name.
'onset_ind' and 'offset_ind' might have to suffice and then for clarity it would need to be clearly defined in docstrings, examples, etc.