Add shock to shock object

Usage

add_shock(
  shock,
  variable,
  value = NA,
  rate = NA,
  absolute = NA,
  start = NA,
  end = NA,
  desc = ""
)

shock: tibble from create_shock()
variable: string variable name
value: numeric, an explicit value or values for the variable, will be extended with last value
rate: numeric, multiplier to influence the original value of the variable
absolute: numeric, absolute value to influence the original value of the variable
start: numeric or date period number for the shock to take place, defaults to NA
end: numeric or date period number for the shock to take place, defaults to NA
desc: string variable description

updated shock object containing added shock