FIX: read_arm_mmcr fails when using a read-only input file (#888)

* FIX: `read_arm_mmcr` fails when using a read-only input file

* FIX: add condition on "heights" dimension

* ENH: I think I have a fix to work for read only.  The proposed method did not work on my mac after downloading data.

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Co-authored-by: AdamTheisen <[email protected]>

Author: isilber

act/io/arm.py

@@ -841,7 +841,7 @@ def read_arm_mmcr(filenames):
     # read it in with xarray
     multi_ds = []
     for f in filenames:
-        nc = Dataset(f, 'a')
+        nc = Dataset(f, 'a', diskless=True)
         # Change heights name to range to read appropriately to xarray
         if 'heights' in nc.dimensions:
             nc.renameDimension('heights', 'range')