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/color.err.t

@@ -45,7 +45,7 @@ if (open my $fh, '>', 'color.rc')
 
 # Test the errors colors
 my $output = qx{../src/task rc:color.rc rc.debug:on add due:__ 2>&1 >/dev/null};
-like ($output, qr/^\033\[33mThe\ date\/duration\ '__'\ was\ not\ recognized.\033\[0m$/xms, 'color.error');
+like ($output, qr/^\033\[33mThe\ duration\ '__'\ was\ not\ recognized\ as\ valid,\ with\ correct\ units\ like\ '3days'\.\033\[0m$/xms, 'color.error');
 like ($output, qr/^\033\[32mTimer\ Config::load\ \(color.rc\) .* \033\[0m$/xms, 'color.debug');
 like ($output, qr/^\033\[34mUsing\ alternate\ .taskrc\ file\ color.rc\033\[0m$/xms, 'color.header');
 like ($output, qr/^\033\[31mConfiguration\ override\ rc.debug:on\033\[0m$/xms, 'color.footnote');