Add macos support

[TIotti]

Small code changes to allow XCode to build the ExodusImport

[NegInfinity]

Hi. I know it has been ages (I apologize for that, there were RL issues), but I cannot accept this pull request as of now.

Moving templated methods to cpp is guaranteed break compilation in situation where *.cpp combining is disabled, as the compiler will be unable to find function bodies. I'll look into other changes you suggested.