Fix for weird dates caused by rounding errors

Use of mjulian2greg.m was generating rounding errors and weird dates.
mjulian2greg.m has now been fixed, so the workaround in
write_FVCOM_elevtide.m and write_FVCOM_forcing.m has been removed.
Minor fix, corrected typo in variable name
Major edits to incorporate Matlab date manipulation functions

Major edits to incorporate Matlab date manipulation functions. Original
file did not use inhouse Matlab date functionality; it generated
rounding errors and weird dates on occasion. Rewritten to use inhouse
Matlab date/time functions, thus avoiding rounding errors.