Initial adding of signal generator classes
Added classes to __init__.py and fixed typos
Changed to use SingleChannelSG class
Fixed single channel SG property
Merge in changes from dev
Changed Holzworth names to match new signal generator contract.
Added apiref for new SigGenABC based Holzworth.
Added newline at end of file
Renamed Channel → SGChannel
Updated remaining Channel refs to SGChannel
Added SignalGenerator classes to instrument.rst docs
Added a few docstrings for SigGen ABCs
Merge branch 'feature-SigGenABC' into dev
Fix import errors
Merge branch 'feature-SigGenABC' into dev
Addressed issue #23

Removes ABCs from root import hierarchy
Updated README.md with link to docs