Bugs

- Added an A3::is_attr type override for 'recur', which needs to be
  treated as an un-expanded 'string' type, most, but not all of the
  time.
- Documented the above in ColRecur.cpp.
- Modified Command.cpp to recognize both type 'duration' and name 'recur'.
- Removed Command::next_mod_group argument coalescing for date types,
  which was not used anyway.
- Improved the error message for unrecognized durations, which previously
  included the word 'date'.
- Modified unit tests to accomodate the above error message change.
- Added bug.972.t unit tests, which fail, because it isn't fixed yet.
- Made A3::dump const so it can be used anywhere.

src/columns/ColRecur.cpp

@@ -39,7 +39,11 @@ extern Context context;
 ColumnRecur::ColumnRecur ()
 {
   _name  = "recur";
+
+  // This is 'string', and not 'duration' to force the value to be stored as a
+  // raw duration, so that it can be reevaluated every time.
   _type  = "string";
+
   _style = "duration";
   _label = STRING_COLUMN_LABEL_RECUR;