Enable multiple filters to be supplied by using get_resource_sql

[Moohan]

I was working with @peterl04. He was using https://www.opendata.nhs.scot/dataset/prescriptions-in-the-community but needed data on specific drugs from a number of practices for ~2 years (24 resources). Each resource takes 30+ seconds to fully download, so filtering is super useful. However, only being able to filter by one drug and practice at a time is quite limiting and makes the code more complicated.

This change worked for me. It just uses get_resource_sql when multiple filters are supplied and generates some appropriate SQL.

I think it could probably do with more tests, especially for incorrect or weird/edge cases. I tried to do it using existing functions as much as possible.

One thing to note is that get_resource_sql is slower than get_resource so there could be a better way of doing this...

[csillasch]

Nice solution!

[Moohan]

@csillasch if you're happy with this approach, would you have time to add any more tests to it?

Also should we have the message? I'm leaning towards removing it, as I'm no longer sure that the user needs to know how we are doing the filtering :D

[csillasch]

Yes sounds good, I will look at adding some more tests.
Agree the user probably doesn`t need to know how the filtering is being done, but might be useful to have a message anyway because it is slower than usual/would be expected?