Improve stimulus waveform synthesis

[swharden]

ABF1 files and ABF 2.9 files aren't generating proper stimulus waveforms (abf.sweepC) at present. The stimulus module is the messiest part of pyABF, and would benefit from a clean re-code.

In the process of recreating this module, ensure ABF1 and ABF2.9 files (created with pCLAMP 11) are supported.

[swharden]

entirely re-wrote the stimulus module, breaking it into two:

• https://github.com/swharden/pyABF/blob/master/src/pyabf/waveform.py
• https://github.com/swharden/pyABF/blob/master/src/pyabf/stimulus.py

Many ABFs which previously produced empty abf.sweepC arrays are now populated. This is evidenced by the fact virtually every ABF on the data page now has a graphed stimulus waveform. Further, many previous stimuli were erroneous (mostly ones with level deltas which did not return to holding levels between sweeps):

• https://github.com/swharden/pyABF/tree/master/data

I'll close this issue when these changes are pushed to pypi.