[WIP] Feature/aps timeseries

[DCMSstats]

Hi Matt,

This might be my last pull request but I might try and update the Rmd file tomorrow with some instructions on how to use + add some tests - will send a follow up email. Just wanted to get the last bits of work I had done so far on here for my successor/you/another to pick up!

Thanks,
Niall

[ivyleavedtoadflax]

Great I'll take a look...If you get time to add tests and some test data, that would really help me get my head around what the functions do!

[ivyleavedtoadflax]

Think probably I would do away with APS_utils.R. The lookup functions probably want to be handled as lookup tables that are bundled as data objects in the package, and the other functions can get rolled into the main utils.R file.