FormatMRC: relax MZ check

newsfragments/740.bugfix

@@ -0,0 +1,2 @@
+``FormatMRC``: relax restrictive check on the overloaded MZ header
+value, which caused failures to read files where MZ == 1

src/dxtbx/format/FormatMRC.py

@@ -63,9 +63,12 @@
 
  # For image stacks, NX==MX etc. should always be true. Assert this
  # to ensure we fail on an MRC file of the wrong type.
- assert hd["nx"] == hd["mx"]
- assert hd["ny"] == hd["my"]
- assert hd["nz"] == hd["mz"]
+ try:
+ assert hd["nx"] == hd["mx"]
+ assert hd["ny"] == hd["my"]
+ assert (hd["nz"] == hd["mz"]) or (hd["mz"] == 1)
+ except AssertionError:
+ raise ValueError("Unexpected data size values in the header")
 
  return hd