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/en-US.h

@@ -600,7 +600,7 @@
 #define STRING_DOM_CANNOT_SET        "DOM: Cannot set '{1}'."
 
 // Duration
-#define STRING_DURATION_UNRECOGNIZED "The date/duration '{1}' was not recognized."
+#define STRING_DURATION_UNRECOGNIZED "The duration '{1}' was not recognized as valid, with correct units like '3days'."
 
 // E9
 #define STRING_E9_UNSUPPORTED        "Unsupported operator '{1}'."