add support for PercentField in orm

[prenit-wankhede]

[gtalarico]

I have a question @prenit-wankhede , not sure if you know -

1. does airtable return percentages as 0-1 or 0-100?
2. does it return floats or Ints?

[larsakerson]

@gtalarico Airtable returns (and takes) percentages as a decimal value between 0 and 1 (optionally including negative values). For the PercentField subclass is to support int values or float values outside that range, shouldn't it convert them to an acceptable format?

@prenit-wankhede would mathematical operations with PercentField values be easier if it consistently returned a float between -1 and 1?