GWHAT crash when date is before 01/01/1900 in weather or water level data files.

[jnsebgosselin]

This is a well known limitation of using the Excel numeric format. Time zero starts at 01/01/1900 and date before this are not supported by the format.

This is well documented here: http://xlrd.readthedocs.io/en/latest/dates.html

So there is a need in GWHAT to go around this limitation.

[jnsebgosselin]