Add install requirements and fix installation errors

[skilkis]

This PR adds installation_requirements to setup.py and fixes the installation errors due to these requirements not being available during install time when setup.py is run. To fix this a static .version file is created, removing the need for having the package version to be specified in init.py of the pyscopus package. Furthermore, setuptools_scm is use to get the latest Git tag when available. Therefore, when the package is installed with a newer Git tag than the static version available in .version; this file will be updated and can later be commited.

Summary of Changes:

• Removed version attribute from init.py
• Added static .version file to contain the current package version
• Removed import of pyscopus in setup.py to fix installation error
• Changed HERE directory to ROOT_DIR and preferred use of Path class over os.path.abspath
• Added setuptools_scm version that uses latest Git tag along with a fallback version from the .version file