Dependency management

• Closes #9 (closed) and #7 (closed)
• Dependencies for miniff are captured in requirements.txt and environment.yml to provide a convenient way to replicate the development environment for pip and conda users respectively.
• Dependency on dfttools is removed.
• Documentation for miniff is hosted on Read the Docs (rtd). The build configuration and dependencies required for rtd to build and host the documentation are specified in the configuration file (.readthedocs.yml)
• Installation steps in the README are updated to reflect the dependency management changes introduced in this branch
• setup.py is updated with minimal set of dependencies needed to build and run miniff. Minimum version required for the dependencies is specified as the current version.