Expose split updates

[mattheaphy]

• expose_split() bug fixes:

  • expose_split() was updated to respect the values of start_date
    and end_date originally passed to the expose() function.
  • Future policy anniversary dates falling on February 29th leap days are now
    consistent with expose()
  • New tests were added to verify that the sum of policy year exposures
    (exposure_pol) after calling expose_split() match exposures produced by
    expose_py().

• The expose() family of functions and add_transactions() now allow date
  columns to be passed as character vectors in YYYY-MM-DD format. Any character
  vectors are converted to dates behind-the-scenes, and any missing values will
  results in an error message.

• News and version update