updated readme to reflect acceptance of excel files in spark_parse

3 small fixes

- change default af_model to spline
- correctly handle when fluorophore calibrant doesn't exist
- correct fluorescein concentrations in claibration plate layout

Merge pull request #4 from ucl-cssb/flow-changes-test

Changes to flow cytometry processing

plate reader calibration workflow upgrades

- seperated and updated parsing of calibration data into spark_parse() from generate_cfs()
- removed hardcoded assumptions about fluorophores from calibration process