Add support for UniBlitz shutter

[prjemian]

USAXS reports they have a UniBlitz shutter. It has two PVs, one to open the shutter and the other to close it. There is no PV for the status.

[prjemian]

Not seeing any such support now in apstools. Tried searching all of GitHub. No obvious determination of state. Only known state is when it has just been set (assume that it has not been changed).

There is no way to determine conclusively the state of this shutter (unless the EPICS database keeps track of it).

[prjemian]

It is trivial to implement controls for each of the PVs but nonsense to make it consistent with the other apstools shutter classes without a clear way to determine the state of the shutter.

[prjemian]

@jkirchman @kmpeters: Is there any state PV? Is it possible to add such a state PV (keeps track of the last OPEN or CLOSE command)?

[prjemian]

Digging deeper through the source code, the ApsPssShutter class matches this exactly:

apstools/apstools/devices/shutters.py

Lines 354 to 367 in 11a9707

	class ApsPssShutter(ShutterBase):
	"""
	APS PSS shutter
	.. index:: Ophyd Device; ApsPssShutter
	* APS PSS shutters have separate bit PVs for open and close
	* set either bit, the shutter moves, and the bit resets a short time later
	* no indication that the shutter has actually moved from the bits
	(see :func:`ApsPssShutterWithStatus()` for alternative)
	Since there is no direct indication that a shutter has moved, the
	``state`` property will always return *unknown* and the
	``isOpen`` and ``isClosed`` properties will always return *False*.

[prjemian]

uniblitz_shutter = ApsPssShutter(
    "",
    name="uniblitz_shutter",
    open_pv=f"{PREFIX}open",
    close_pv=f"{PREFIX}close"
)

Apparently, both open_pv and close_pv are complete PV names.

[prjemian]

Note that the support uses a fixed value of 1 when writing to then open:

apstools/apstools/devices/shutters.py

Line 451 in 11a9707

self.open_signal.put(1)

and close PV:

apstools/apstools/devices/shutters.py

Line 464 in 11a9707

self.close_signal.put(1)

The base class supports custom values for self.open_value and self.close_value:

apstools/apstools/devices/shutters.py

Lines 87 to 88 in 11a9707

	open_value = 1 # value of "open"
	close_value = 0 # value of "close"

The base class constructor does not provide kwargs to set these. Might be a good addition, otherwise need to set shutter.close_value=1 once the shutter object is created.

[prjemian]

Add to documentation that ApsPssShutter can also be used for the UniBlitz shutter.