Add ReadOptions implementation

[y1zhou]

This is to address #125, where we pass in ReadOptions to the open() functions instead of just StrictnessLevel for future extensibility.

• [x] PDB parser with ReadOptions
• [x] mmCIF parser with ReadOptions
• [x] Route to open_gz when options.decompress is true
• [x] Remove hydrogens from the model when options.discard_hydrogens is true
• [x] Skip other models when options.only_first_model is true
• [x] Only parse ATOM records when options.only_atomic_coords is true

Some things up for discussion:

1. What is the intention of capitalise_chains? If I recall correctly all uppercase letters, lowercase letters, and numbers are valid chain identifiers.
2. Can I modify the signatures of open_pdb_raw and open_mmcif_raw? They are public functions but it seems they are only meant to be called by open_pdb and open_mmcif. If we maintain open_pdb_raw_with_options and open_mcif_raw_with_options there's going to be a lot of duplicated code (or we extract common methods into another core function).