PVCamera: default again to TriggerMode.ONCE (fixup 35173b0)

When we subclassed from TriggerTargetMixin we messed up the correct
mapping of trigger modes.  The PVCAM trigger mode strobed is not the
same as Microscope's strobed mode and is actually similar to our once.
Fix the mapping which effectively fixes the default.

Author: carandraug

NEWS.rst

@@ -17,6 +17,12 @@ Version 0.7.0 (upcoming)
 
   * Toptica iChrome MLE
 
+* Device specific changes:
+
+  * `PVCamera`: in version 0.6.0 the default PVCAM trigger mode was
+    accidentally changed from "strobed" to "trig. first".  This has
+    now been reversed.
+
 * The device server logging was broken in version 0.6.0 for Windows
   and macOS (systems not using fork for multiprocessing).  This
   version fixes that issue.

microscope/cameras/pvcam.py

@@ -1172,13 +1172,19 @@ def __repr__(self):
 
 PV_MODE_TO_TRIGGER = {
     TRIG_SOFT: (microscope.TriggerType.SOFTWARE, microscope.TriggerMode.ONCE),
+    # Microscope and PVCam use mode strobe for very different things,
+    # check with the PVCam manual carefully.  PVCam's STROBED_MODE
+    # means that one external trigger starts *each* exposure in a
+    # sequence, which maps to Microscope trigger mode ONCE.  PVCam's
+    # TRIGGER_FIRST_MODE means that one external trigger signals the
+    # start of a sequence.
     TRIG_FIRST: (
         microscope.TriggerType.RISING_EDGE,
-        microscope.TriggerMode.ONCE,
+        microscope.TriggerMode.START,
     ),
     TRIG_STROBED: (
         microscope.TriggerType.RISING_EDGE,
-        microscope.TriggerMode.STROBE,
+        microscope.TriggerMode.ONCE,
     ),
     TRIG_BULB: (
         microscope.TriggerType.RISING_EDGE,