Add SWEEP_CONTROL class

SWEEP_CONTROL can be used to build sweep control widgets that
give control over the basic carrier frequency sweep functionality
of a function generator.

Currently implemented after the Sweep interface for the Marconi
Server.
Change units on sweep time/step, separate SweepRange methods

Base units for sweep time as milliseconds, and for step its khz.
This is to make it easier to choose good values for a smooth sweep.

SweepRange was split into independent SweepRangeStart and
SweepRangeStop methods. This independence makes programming the
GUI interface simpler.
SWEEP_CONTROL UI almost completed, still needs update commands

Still needs onChange style update methods, but most of the GUI
interface is finished.
SWEEP_CONTROL, a little more work on GUI layout
Fix sweep start/stop freq limits
SWEEP_CONTROL: implement main slots, needs testing

Implemented slots for carrierMode, etc. but not sure if these
will work properly. Needs testing.
SWEEP_CONTROL updates state properly on start
MarconiServer: create "hidden" ver of all settings

Settings are not accesible from within the server, because a context
cannot be passed in. Creating non-setting methods, and then calling
those from within the settings, makes all functionality accessible
to the server, with only settings available to the client.
SWEEP_CONTROL: ready to test CarrierMode button

All other buttons should not cause problems.
SWEEP_CONTROL: carrierModeButton.state now string

In makeCarrierModeButton the state is now FIXED or SWEPT rather than
boolean valued. This is slightly inconvenient but keeps things
consistent.
Modify Marconi2024_Server.py, fix yield stmt
Fix syntax errors in SWEEP_CONTROL.py

The sweep control widget will run when called directly from python,
the labrad manager recognizes a new client (from the connectAsync),
but the QtMainWindow for the widget does not show up.

Similar actions for the TRAPDRIVE_CONTROL and TICKLE_CONTROL allow
the window to show up.

Possible source of problem: reactor already installed error.
Merge branch 'master' into gui_dev
SWEEP_CONTROL reactor problem fixed
Merge branch 'gui_dev2' into gui_dev