Create utility functions to help with stacking/merging data domains

[jwildfire]

Create utility functions for combining data across domains:

• stack_measures
  • stacks labs, vs, ecg and other domains containing biometric measures.
  • required columns to be renamed/stacked are: id_col, value_col, measure_col, visit_col.
  • optional columns are normal_col_high, normal_col_low, studyday_col, visitn_col, unit_col
• stack_events
  • stacks aes, cm, mh, exp and other domains containing events.
  • required columns: id_col, stdy_col, endy_col,
  • optional columns vary by domain. Likely just want to add a "details" column that varies by domain.
• merge_dm
  • right_joins specified columns from dm to the current data by usubjid