Export CMake targets so build directory can be used directly

[ZedThree]

• Add export(EXPORT ...) to allow the build directory to be used directly by another project. Also necessary to allow FetchContent to work. Might help with CMake FetchContent Module #97
• Change CMAKE_SOURCE_DIR to CMAKE_CURRENT_SOURCE_DIR, necessary for building from another project
• Add the alias netCDF::netcdf-cxx4, which makes find_package/add_subdirectory both expose the same target name

[WardF]

Thanks very much!

[brhillman]

Fixes #101