Add typequote argument to control whether DX array type should be quoted

This change works around another example of poorly hard-coded DX parser
(see also issue MDAnalysis#35).

See the code below from the GridForcesGrid.C file in NAMD:
fscanf(poten_fp, "object %*d class array type double rank 0 "
                 "items %*d data follows\n");
which will only work if the quote character (introduced in 115b05a to appease
the PyMol parser) is omitted.

gridData/OpenDX.py

@@ -301,7 +301,8 @@ class array(DXclass):
         # "string" not automatically supported
     }
 
-    def __init__(self, classid, array=None, type=None, **kwargs):
+    def __init__(self, classid, array=None, type=None, typequote='"',
+                 **kwargs):
         """
         Parameters
         ----------
@@ -347,6 +348,7 @@ def __init__(self, classid, array=None, type=None, **kwargs):
                                   "types are: {1}".format(type,
                                                           list(self.dx_types.values()))))
             self.type = type
+        self.typequote = typequote
 
     def write(self, file):
         """Write the *class array* section.
@@ -367,9 +369,10 @@ def write(self, file):
                               "Supported valus are: {}\n"
                               "Use the type=<type> keyword argument.").format(
                                   self.type, list(self.dx_types.keys())))
+        typelabel = (self.typequote+self.type+self.typequote)
         DXclass.write(self,file,
-                      'type "{0}" rank 0 items {1} data follows'.format(
-                          self.type, self.array.size))
+                      'type {0} rank 0 items {1} data follows'.format(
+                          typelabel, self.array.size))
         # grid data, serialized as a C array (z fastest varying)
         # (flat iterator is equivalent to: for x: for y: for z: grid[x,y,z])
         # VMD's DX reader requires exactly 3 values per line

gridData/core.py

@@ -405,7 +405,7 @@ def _load_plt(self, filename):
         grid, edges = g.histogramdd()
         self.__init__(grid=grid, edges=edges, metadata=self.metadata)
 
-    def export(self, filename, file_format=None, type=None):
+    def export(self, filename, file_format=None, type=None, typequote='"'):
         """export density to file using the given format.
 
         The format can also be deduced from the suffix of the filename
@@ -424,10 +424,13 @@ def export(self, filename, file_format=None, type=None):
 
         Parameters
         ----------
+
         filename : str
             name of the output file
+
         file_format : {'dx', 'pickle', None} (optional)
             output file format, the default is "dx"
+
         type : str (optional)
             for DX, set the output DX array type, e.g., "double" or
             "float"; note that PyMOL only understands "double" (see
@@ -437,12 +440,19 @@ def export(self, filename, file_format=None, type=None):
 
             .. versionadded:: 0.4.0
 
-
         .. _`#35`: https://github.com/MDAnalysis/GridDataFormats/issues/35
 
+        typequote : str (optional)
+            For DX, set the character used to quote the type string;
+            by default this is a double-quote character, '"'.
+            Custom parsers like the one from NAMD-GridForces (backend for MDFF)
+            expect no quotes, and typequote='' may be used to appease them.
+
+            .. versionadded:: after 0.4.0
+
         """
         exporter = self._get_exporter(filename, file_format=file_format)
-        exporter(filename, type=type)
+        exporter(filename, type=type, typequote=typequote)
 
     # note: the _export_FORMAT() methods all take the filename as a mandatory
     # argument. They can process kwargs but they are not required to do
@@ -461,7 +471,7 @@ def _export_python(self, filename, **kwargs):
         with open(filename, 'wb') as f:
             cPickle.dump(data, f, cPickle.HIGHEST_PROTOCOL)
 
-    def _export_dx(self, filename, type=None, **kwargs):
+    def _export_dx(self, filename, type=None, typequote='"', **kwargs):
         """Export the density grid to an OpenDX file.
 
         The file format is the simplest regular grid array and it is
@@ -495,7 +505,7 @@ def _export_dx(self, filename, type=None, **kwargs):
             positions=OpenDX.gridpositions(1, self.grid.shape, self.origin,
                                            self.delta),
             connections=OpenDX.gridconnections(2, self.grid.shape),
-            data=OpenDX.array(3, self.grid, type=type),
+            data=OpenDX.array(3, self.grid, type=type, typequote=typequote),
         )
         dx = OpenDX.field('density', components=components, comments=comments)
         dx.write(filename)