Implement an overload version of swapEndianness() function.

The double version of the function swaps bytes 0,1,2,3,4,5,6,7 to 3,2,1,0,7,6,5,4.
This swaping is used for double_precision format of Gocad.
Speed up Mesh::removeUnusedMeshNodes() a bit.
Added some output for user information.
Changed the data type that holds the flags information.

If the Gocad SGrid files use double_precision the flags are also stored with 64 bits.