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 (closed)
• 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)

Closes #56 (closed)