update did error handling of time variables

[s3alfisc]

• drop error message that hints that yyyymmdd format was required - it is not.
• update docstrings of all did methods.
• add check that gname variable in the set of values of tnames and 0.
• check that tnames does not contain a 0, this is reserved for the gname group.

[escherpf]

For me the problem wasn't so much the way errors were handled as much as did supporting variables to be able to work more easily with year-month variables, but it seems that is a bigger lift. A workaround for the user would be to recast the year-month variable into a relative time (i.e., an integer), which is essentially what is done with did2s.