UDUNITS2 correct bad timestamp clock time.

This PR performs a convenience correction of a units string containing an incomplete timestamp clock.

The UDUNITS2 grammar specifies that at a minimum the clock of a timestamp must include the hours **and** minutes i.e. ```<hour> ":" <minute> (":" <second>)?``` see http://www.unidata.ucar.edu/software/udunits/udunits-2/udunits2lib.html#Grammar.

i.e. UDUNITS2 interprets the following incomplete timestamp ```hours since 1970-01-01 9``` as ```hours since 1970-01-01 00:00:00``` i.e. the incomplete clock time is silently ignored and the timestamp defaults to midnight.

This PR circumvents user confusion, thus ```hours since 1970-01-01 9``` is corrected to ```hours since 1970-01-01 09:00:00```.