Filtering and stratification steps

[tbepler]

Filtering:

1. Time filter, -8 to +48 hours
2. ICU sepsis
3. Age (18+), any other demographic filtering
4. Most recent echo

Stratification (aka features of interest):

1. Mechanical ventilation during echo, type of ventilation
2. Creatinine at admission to ICU
   ... see Build echo features table #3