- 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.
