Simplify input data format

[hillalex]

Proposed standard input data format:

• id
• time
• biomarker
• value
• ... covariates

[hillalex]

Partially addressed in #11 where the inputs have been simplified to:

• pid
• date
• last_exposure_date
• titre_type
• value
• censored
• ... covariates

The next step will be to allow date/last_exposure_date to be relative instead of absolute dates, see comment: https://github.com/seroanalytics/epikinetics/pull/11#issuecomment-2391314509

[hillalex]

Closed by #16