Epoch functions

[dmcghan]

Alternative implementation. Removed timestamp2epoch in favor of implicit data conversion. Here, date2epoch uses the timestamp data type so if a date is passed it doesn't lose precision but any timestamp data type can be passed without losing milliseconds.

Also, both functions now have an additional tzr parameter which allows for additional flexibility when needed. In the case of date2epoch, the parameter can be used to specify which timezone the date or timestamp being passed in is from. For epoch2date, the tzr parameter allows the user to get a date back in a specific timezone.

The tzr parameter defaults to the sessiontimezone for convenience.

[janihur]

Thank you ! This contribution is welcome, but unfortunately @martindsouza has not have time for the project recently. Please be patient !