FormatROD set beam probe to electron for ED (cctbx#728)

* Use a short-wavelength heuristic to set beam probe to electron.

The CAP format does actually have a header value to identify an
ED experiment (see b_is_microed_experiment in sources). However
we do not have a complete header reader in this format class so
we cannot reliably find this value.

newsfragments/728.bugfix

@@ -0,0 +1 @@
+``FormatROD``: set the beam probe to "electron" for 3D ED experiments.

src/dxtbx/format/FormatROD.py

@@ -25,6 +25,7 @@
 
 from dxtbx.ext import uncompress_rod_TY6
 from dxtbx.format.Format import Format
+from dxtbx.model.beam import Probe
 
 
 class FormatROD(Format):
@@ -283,11 +284,19 @@ def get_beam(self, index=None):
         return Format.get_beam(self)
 
     def _beam(self):
+
+        wavelength = self._bin_header["alpha1_wavelength"]
+        if wavelength <= 0.05:
+            probe = Probe.electron
+        else:
+            probe = Probe.xray
+
         return self._beam_factory.make_polarized_beam(
             sample_to_source=(0.0, 0.0, 1.0),
-            wavelength=self._bin_header["alpha1_wavelength"],
+            wavelength=wavelength,
             polarization=(0, 1, 0),
             polarization_fraction=0.5,
+            probe=probe,
         )
 
     def get_detector(self, index=None):