Specifiy the version numbers/commit hashes and fix setup.py

[jkretz]

Closes #94 and closes #95

This PR addresses two issues:

• The version numbers/commit of packages needed to run the checker are now specified. This prevents that a downstream change in a requited package will break the checker.
• Furthermore, this PR fixes the setup.py so that one can install the checker from the remote repo without the need to clone it. As not all the requirements can be solved via pip (i.e. udunits), there still is a combination of pip and conda to install the necessary packages. The installation instructions for installing the checker from the remote repo via pip are as follows:

  conda create --name atmodat python=3.8 pip udunits
  conda activate atmodat
  pip install git+https://github.com/AtMoDat/atmodat_data_checker@set_version_hash

  @wachsylon maybe you can have a look at that?

[jkretz]

I will merge this for now as the changes to setup.py are needed in #97 as well. For now, I won't amend the installation instructions, as I plan to revise them later when addressing #93.