Work on ConceptSet Class

[mdlavallee92]

• add TimeWindows class
• add class methods to grab elements
• some minor folder restructuring
• some minor function reorg

From this PR the following will work on a ConceptSetDefinition Line Item:

library(Capr)
cs1 <- list(
  't2d' = cs(descendants(201826), name = "t2d"),
  'ckd' = cs(descendants(46271022), name = "ckd")
)

hh <- createConceptSetLineItem(
  name = "Baseline Conditions",
  ordinal = 1,
  statistic = createPresence(),
  timeWindows = createTimeWindows(
    timeInterval(lb = -365, rb = -1),
    timeInterval(lb = 0, rb = 90)
  ),
  conceptSets = cs1,
  domain = "condition_occurrence"
)

hh$getTimeWindows()
hh$getConceptSets()
hh$getDomain()
hh$getDefinitionType()
hh$getStatisticType()

[mdlavallee92]

Additional Updates:

• Convert ConceptSetDefinitionLineItem to singleton class. only accepts one ConceptSet and one TimeInterval
• Update ConceptSet batch add to work with purrr
• Update class methods with modifications to CS class
• Start adding code for Demographics (Age, Concept, Year).....added class, ui, and sql