File format handling

[jnooree]

This is a tracking issue for the File format handling milestone.

### Format parsing
- [ ] https://github.com/seoklab/nurikit/issues/78
- [ ] https://github.com/seoklab/nurikit/issues/77
- [ ] https://github.com/seoklab/nurikit/issues/162
- [ ] https://github.com/seoklab/nurikit/issues/241
- [ ] FASTA parser
- [ ] https://github.com/seoklab/nurikit/pull/444
- [ ] https://github.com/seoklab/nurikit/pull/445

### Format writing
- [ ] https://github.com/seoklab/nurikit/issues/273
- [ ] https://github.com/seoklab/nurikit/issues/249
- [ ] https://github.com/seoklab/nurikit/issues/258
- [ ] FASTA writer
- [ ] https://github.com/seoklab/nurikit/pull/463
- [ ] PDBx/mmCIF writer