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.

test/feature.1013.t

@@ -43,9 +43,9 @@ if (open my $fh, '>', 'outerr.rc')
 
 # Check that errors are sent to standard error
 my $stdout = qx{../src/task rc:outerr.rc add due:__ 2> /dev/null};
-unlike ($stdout, qr/^The date\/duration '__' was not recognized.$/ms, 'Errors are not sent to stdout');
+unlike ($stdout, qr/^The duration '__' was not recognized as valid, with correct units like '3days'.$/ms, 'Errors are not sent to stdout');
 my $stderr = qx{../src/task rc:outerr.rc add due:__ 2>&1 >/dev/null};
-like ($stderr, qr/^The date\/duration '__' was not recognized.$/ms, 'Errors are sent to stderr');
+like ($stderr, qr/^The duration '__' was not recognized as valid, with correct units like '3days'.$/ms, 'Errors are sent to stderr');
 
 # Check that headers are sent to standard error
 $stdout = qx{../src/task rc:outerr.rc list 2> /dev/null};