ETL updates

[mikkokotila]

• [x] Let's filter the dataset down to ages 15-100 only.
• [x] 't' and 'f' replaced 1 and 0 (to minimize ETL)
• [x] lower_case column names for everything
• [x] should [79,98) in age_group be [79,98]?
• [x] add new concept "total" which is simply 1 for every record (for groupby use)
• [x] pre-sort by age-group with youngest first
• [x] change name of file to ICU_burden_dataset.csv