Fixing UVCDAT runtime environment

Sourcing the setup_environment.sh before using the software is error-prone. Furthermore, it is not actually needed anymore when running 'uvcdat', since the wrapper script will source it; this causes a warning to be displayed (`UVCDAT setup already sourced for this install location.`).

Also, once `setup_runtime.sh` is sourced, a whole bunch of executables appear in the path, replacing the system's default (things like `python`, `curl`, `uuid`... 260 in total on my system). For the same reasons, installing UV-CDAT globally is not a good idea (will break system's `python`).

This pull request intends to make the setup_runtime logic internal, and exposes `uvcdat` and `uvcdatpython` wrappers that can safely be added to the PATH (since they are on a directory of their own).
I believe we will need UVCDAT_INSTALL_PREFIX for post-install script. Do we really need to take it out?