Support string-like variables in DataFile

BOUT++ may now write 'string' variables. In NetCDF files this have a
'char<n>' dimension, where '<n>' is the length of the dimension. In HDF5
files they should have an attribute bout_type="string" or
bout_type="string_t" (for time-dependent arrays of strings).

boututils/datafile.py

@@ -470,6 +470,13 @@ def dimlen(d):
 
     def _bout_type_from_dimensions(self, varname):
         dims = self.dimensions(varname)
+
+        if any("char" in d for d in dims):
+            if 't' in dims:
+                return "string_t"
+            else:
+                return "string"
+
         dims_dict = {
             ('t', 'x', 'y', 'z'): "Field3D_t",
             ('t', 'x', 'y'): "Field2D_t",
@@ -484,7 +491,24 @@ def _bout_type_from_dimensions(self, varname):
 
         return dims_dict.get(dims, None)
 
-    def _bout_dimensions_from_type(self, bout_type):
+    def _bout_dimensions_from_var(self, data):
+        try:
+            bout_type = data.attributes["bout_type"]
+        except AttributeError:
+            defdims_list = [(),
+                            ('t',),
+                            ('x', 'y'),
+                            ('x', 'y', 'z'),
+                            ('t', 'x', 'y', 'z')]
+            return defdims_list[len(np.shape(data))]
+
+        if bout_type == "string_t":
+            nt, string_length = data.shape
+            return ('t', "char" + str(string_length),)
+        elif bout_type == "string":
+            string_length = len(data)
+            return ("char" + str(string_length),)
+
         dims_dict = {
             "Field3D_t": ('t', 'x', 'y', 'z'),
             "Field2D_t": ('t', 'x', 'y'),
@@ -541,15 +565,7 @@ def write(self, name, data, info=False):
             # Not found, so add.
 
             # Get dimensions
-            try:
-                defdims = self._bout_dimensions_from_type(data.attributes['bout_type'])
-            except AttributeError:
-                defdims_list = [(),
-                                ('t',),
-                                ('x', 'y'),
-                                ('x', 'y', 'z'),
-                                ('t', 'x', 'y', 'z')]
-                defdims = defdims_list[len(s)]
+            defdims = self._bout_dimensions_from_var(data)
 
             def find_dim(dim):
                 # Find a dimension with given name and size
@@ -786,11 +802,13 @@ def dimensions(self, varname):
             "FieldPerp_t": ('t', 'x', 'z'),
             "Field2D_t": ('t', 'x', 'y'),
             "scalar_t": ('t',),
+            "string_t": ('t', 'char'),
             "Field3D": ('x', 'y', 'z'),
             "FieldPerp": ('x', 'z'),
             "Field2D": ('x', 'y'),
             "ArrayX": ('x',),
             "scalar": (),
+            "string": ('char',),
         }
         try:
             return dims_dict[bout_type]
@@ -892,7 +910,7 @@ def write(self, name, data, info=False):
             print("Creating variable '" + name +
                   "' with bout_type '" + bout_type + "'")
 
-        if bout_type in ["Field3D_t", "Field2D_t", "FieldPerp_t", "scalar_t"]:
+        if bout_type[-2:] == "_t":
             # time evolving fields
             shape = list(data.shape)
             # set time dimension to None to make unlimited