Allow two-part julian dates for extra precision
Test all combinations of scalar/array tdb and tdb2

Also, renamed first argument from `tdb1` back to `tdb` to avoid breaking
any existing user code that happened to use a keyword argument to
specify `tdb`.
Avoid extra `tdb + tdb2` with `index` sanity-check
Slight reformat and reword of method docstrings
Merge branch 'feature/dual-jd'; thank you, Marten!