Made size of the char array c dependent to the size of the type.
Implement an overload version of swapEndianess() 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.
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.
Speed up Mesh::removeUnusedMeshNodes() a bit.