policies_simulate: Simulate a data frame of policies
In PirateGrunt/imaginator: Simulate General Insurance Policies and Losses

Given a starting number of policies, this function will generate additional years of policy data.

Growth is given as a the positive rate of growth of new policies. This may be set to zero.

Retention is given as the portion of expiring policies which will renew.

policies_simulate(
  n,
  policy_years,
  num_years,
  exposure = 1,
  retention = 1,
  growth = 0,
  start_id = 1,
  additional_columns
)

`n`	An integer giving the number of policies in the first year
`policy_years`	A vector of integers in sequence
`num_years`	The number of years to simulate. If 'policy_years' is given, this is ignored.
`exposure`	Exposure per policy
`retention`	A vector indicating loss of policies
`growth`	A vector indicating the rate of growth of policies
`start_id`	Integer of the first number in the policy ID sequence
`additional_columns`	A list of addtional column names and values