Initial implementation of search API

[stuartlynn]

• [x] define a structure to represent a search query
• [x] map cli options to that struct
• [x] write functions to execute the search and return the results

[penelopeysm]

Okay, I don't think this is perfect but I think it's at a stage where we could consider merging. Will self-review and drop some comments.

As a general comment, to make the diffs a bit easier to see, I opened a separate draft PR which is (mostly!) limited to the changes to the search bits relative to the last time we worked on it together on Tuesday. It may or may not be useful. https://github.com/Urban-Analytics-Technology-Platform/popgetter-cli/pull/41/files

[penelopeysm]

Ok, merged main back in. That was a fun exercise!

[penelopeysm]

I think all the outstanding things to do have been converted to issues so will merge now!