get Snakemake input from LabKey API

- add script that prepares Snakemake input files 'samples.tsv' and 'config.yaml' from LabKey table
- script either connects to API directly (with '--remote' and related options) or processes a tab-separated LabKey dump file
- add tests for both use cases
- common input files for tests now in 'tests/input_files'
- update all other tests to account for new file locations
- update documentation