mapOverlay updated and bug fixed

Variable name `Basemap` has been changed to `mapObj` to align with the rest of the package.

More importantly:
Previous operation of the code was to set `dateTime` to the current time if the option was not set.  This has been changed to be set to `mapObj.dateTime` if the option is not set.  The old way could result in unexpected behaviour, with the radar/FOV ending up in the wrong place when the map object was not at the same time as the current time.  Now it will end up at the correct location by default, and if supplied with a different time for whatever reason will use that.

This is not strictly related to coord_conv or MLT, as it turns out, but is more of a bugfix.