phsmethods 0.2.1

• Three functions renamed to improve code clarity: postcode() to format_postcode(); age_group() to create_age_groups(); fin_year() to extract_fin_year(). The old functions will still work but will produce a warning. After a reasonable amount of time they will be removed completely.

• New functions added:
  age_calculate()(#65, @Nic-chr);
  dob_from_chi()(#42, @Moohan); and
  age_from_chi()(#42, @Moohan)

• Change the output for chi_check so that empty string ("") reports as missing (#76)