Support @DATE@, @TIME@ and @DATETIME@ to insert values after sub- and ses- values #605
Description
Further to #510 (also discussed here) it would be nice to support outright the sub-<date> (or ses-, <time>, <datetime>) format. The two things to do are filter them as part of #510 and patch @DATE@, @TIME@ and @DATETIME@ to insert correctly in after sub- and ses- keys.
Otherwise, these should work outright. The only other issues is that for @DATETIME@ you would need to set allow_letters_in_sub_ses_values which could be confusing, but I think we can handle that if it is raised as an issue.