Auto detect netcdf c++ lib if installed via conda

The conda netcdf-cxx4 library has been updated quite some time ago, and it can be used in the build process instead of compiling netcdf4 manually. Minor changes in the cmake file were neccessary.

Note that if this gets merged, the documentation needs to be adapted:

• The conda package for netcdf-cxx4 (4.3.1 required!) needs to be added: conda install -c conda-forge "netcdf-cxx4>=4.3.1"
• The manual compilation step needs to be removed.
• Since the default conda solver takes a while to solve these dependencies, we could provide a pre-solved environment. Mine looks like this: met3d.yml. Then, the user can create the environment from this file, so we don't need this long installation command anymore. This also makes sure, that the we and the end-user have exactly the same package versions if there are issues.

Adresses issue #294 (closed)