Add constructor to correlator

Initializes filter list to list so that GUI doesn't break with a new
filter.

--CAR and BRJ
Really make sure sequence is long enough for marker

--CAR
Hard code config.json config file in PyQLab root folder.

Removed dependency on PYQLAB_CFGFILE environment variable.
Make Channels runnable on its own.

Useful for testing.
Add HeterodyneFrequency view.

--BRJ
Swap labels of RF and LO in heterodyne view.

--HP
Fix for running config.py from a different folder.

--BRJ
plot_pulse_files now accepts .awg files.

Update read_Tek_awg_file to properly build the sequence table, and use it in plot_pulse_files.
Put empty string in for quickPickFile if it doesn't exist.
Fix wfLib compression when SSB is turned on

Compress *after* applying SSB to remove baseband pulses from the
waveform library.

--CAR
Previous commit accidently committed a commenting out of slave trigger line.
Update README to reflect configuration changes.
Create config file from example if it doesn't exist.
Better path handling for AWG filenames
Reorganize toolbar into a menu.

Moves quick picker into the 'File' menu. 'Apply' button is now saves the
settings to the current settings file.
Add cancel buttom to QuickPickView and rearrange into two rows.
Bugfixes from testing.

--BRJ
Rename and reorder file menu.

I think these names make a little more sense.
Merge branch 'develop' of github.com:BBN-Q/PyQLab into develop
Change Instrument super-class over to Atom

Also change name -> label.