Add finance and reverse to format_ay()

[cjrace]

Is your feature request related to a problem? Please describe.

Currently we only allow for converting 6 digit numbers into academic year format, there's room to build on this to make the function more useable.

Describe the solution you'd like Add:

• type argument, with options 'academic' and 'finance'
• reverse argument for inverting the process

Rename the function to account for both types of year, or create a second function for FY.

Describe alternatives you've considered No other alternatives considered yet.

Additional context Financial years would format as 2016-17