Resolves issue #102

The two methods

• P1Function::assign( P2Function, ... )
• P2Function::assign( P1Function, ... ) are replaced by the free functions
• P1ToP2Conversion()
• P2ToP1Conversion() For lack of a better idea I placed them in hyteg/src/gridtransferoperators. That seemed at least partially fitting and better than any available alternative.