Fix bit depth for Eiger / NXmx for i19-2 (#652)

Fix bit depth for Eiger / NXmx for i19-2

Add bit depth check from image metadata as implemented on other Diamond NXmx
installations. Fixes dials/dials#2473

newsfragments/652.bugfix

@@ -0,0 +1 @@
+``dxtbx``: add fix for Eiger / NXmx data from i19-2 to correctly assign the image bit depth

src/dxtbx/format/FormatNXmxDLSI19_2.py

@@ -8,7 +8,7 @@
 from libtbx import Auto
 
 from dxtbx.format.FormatNXmx import FormatNXmx
-from dxtbx.format.FormatNXmxDLS import FormatNXmxDLS
+from dxtbx.format.FormatNXmxDLS import FormatNXmxDLS, get_bit_depth_from_meta
 from dxtbx.masking import GoniometerMaskerFactory
 
 
@@ -41,6 +41,11 @@ def __init__(self, image_file, **kwargs):
         """Initialise the image structure from the given file."""
         self._dynamic_shadowing = self.has_dynamic_shadowing(**kwargs)
         super().__init__(image_file, **kwargs)
+        try:
+            self._bit_depth_readout = get_bit_depth_from_meta(self._meta)
+        except Exception:
+            self._bit_depth_readout = 16
+
 
     def get_goniometer_shadow_masker(self, goniometer=None):
         """Apply the dynamic mask for a diamond anvil cell with a 76° aperture."""