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.
Merge pull request #3 from kjamoudry/master

Merge fixes from Karen Amoudry to correct some rounding errors in the Times and Itime2 variables in the forcing files. 

In particular, there are significant changes to the greg2mjulian (and mjulian2greg?) function to use native MATLAB functions (datenum et al.) rather than the MATLAB Central third-party greg2julian function. 

This also includes some changes which I (Pierre Cazenave) made to the code in parallel to Karen's changes (though on different functions).
Bring in changes from Karen Amoudry following her fixes for the rounding issues in the Times and Itime2 variables.