A refactor and creation of create_custom_centile function
2.5.0
New features: 1. create_chart now accepts a custom list of centiles as well as the standard cole-nine-centiles and three-percent-centiles 2. the generate_centile function has been exposed as a public function *Fix* 3. the advice strings relating to age calculation refer to 'child' rather than 'baby'
2.4.3
a-wei-0513 changed the centile_band function to accept different centile formats. So far only 2 are supported: the Cole 9 centile format where each centile is 2/3 of an SD, and the standard 3/5/10/25/50/75/90/95/97 format as used by CDC.