Allow users to set NetCDF_[C|Fortran]_PATHS

Currently users are recommended to use NetCDF_PATH or NetCDF_[C|Fortran]\_PATH to set the paths to the NetCDF libraries. Although NetCDF_[C|Fortran]\_PATHS is recognized by the find_package_component() cmake function (in cmake/LibFind.cmake), that is used to find the NetCDF library/package, there are assumptions in the code that its never set by the user (the user invoking cmake).

In FindNetCDF.cmake NetCDF_[C|Fortran]\_PATHS is initialized (in certain conditions). Instead of initializing NetCDF_[C|Fortran]\_PATHS directly a temporary variable could be initialized and appended to NetCDF_[C|Fortran]\_PATHS ensuring that the paths set by the user is not overridden.

Also see Issue #456