ENH: add a pause-on-interrupt suspender variant

[tacaswell]

Demo of how this works:

import ophyd
from ophyd import EpicsSignal

# ophyd.set_cl('caproto')

from bluesky import RunEngine
import bluesky.plan_stubs as bps
from bluesky.utils import ts_msg_hook
from bluesky.suspenders import SuspendBoolHigh
from functools import partial

ev = EpicsSignal('simple:A', name='a')

class PauseInterupt(SuspendBoolHigh):

    def __call__(self, value, **kwargs):
        """Make the class callable so that we can
        pass it off to the ophyd callback stack.

        This expects the massive blob that comes from ophyd
        """
        with self._lock:
            if self._should_suspend(value):
                self._tripped = True
                loop = self.RE._loop
                # this does dirty things with internal state
                if (self.RE is not None):
                    print('entered right condition')
                    cb = partial(
                        self.RE.request_pause, defer=False)
                    if self.RE.state.is_running:
                        print('about to pause')                        
                        loop.call_soon_threadsafe(cb)
                        self._SuspenderBase__make_event()
            elif self._should_resume(value):
                self._tripped = False
                self._SuspenderBase__set_event()                

RE = RunEngine()
RE.msg_hook = ts_msg_hook

def manual_plan():
    yield from bps.sleep(.3)
    print('go flip the PV manually')
    yield from bps.sleep(100)

[prjemian]

Thanks!