allow large units to r-click open doors

This simple change allows tanks to open doors by approaching them, then r-clicking beyond them.
It appeared to be deliberate not to allow tanks to open doors this way, so there may be a reason this was not enabled.  However, with this change I tested door-opening behavior successfully for both double-width and single-width doors.

This does not change their already-successful ability to open doors as a precursor to passing through them, which they have always been able to do.

Fixes issue 732  http://openxcom.org/bugs/openxcom/issues/732