This commit removes the original member functions
void Tables::addPVTO()
void Tables::addPVTW()
void Tables::addPVTG()
since these are no longer needed. The PVT tables are now defined in
terms of the 'addPVTTables()' member function
While here, also remove the fwrite() free function which was defined
in terms of LibECL types.
Update the unit test accordingly.