install dependencies via Conda

- add 3 separate Conda environment files for installing
  - essential dependencies without Singularity (most users)
  - essential dependencies including Singularity (root users on Linux only)
  - non-essential dependencies (required for tests etc.)
- install Python dependencies via `pip` _within_ Conda environment to avoid #56
- update Conda-based installation instructions
  - describe usage of different environment files
  - add supported OS versions (macOS not yet added, because only installation works)
- update CI config to use same setup (based on Miniconda Docker image)