read/write locations.geojson and rework spec implementation

[polettif]

As discussed in #36, with this PR import_gtfs() can read feeds containing locations.geojson files. Changes:

• read_files() now reads a file as json if it ends with ".geojson"
• added dependency on jsonlite for read_json()
• added file_ext for each file to gtfs_standard

There are no interface changes except for a slightly changed error message.

Geojson files are not validated further (except maybe what happens within read_json). It's a rather minimalistic approach to solve the issue. I hope I used the right patterns and syntax, I got a bit confused with the handling of file variables with extensions and those without, I'm not sure it's always clear what's expected.

[polettif]

Since there have been quite some changes and expansions with GTFS in the past few years – and it looks like there are still some underway – we reached to a limit with hard coding the specifications in get_gtfs_standards(). I implemented a method to parse the reference.md file and document which "gtfs reference types" are converted to which data.table type in the new exported gtfs_reference dataset. Parsing is done manually in inst/reference so it doesn't happen when you build the package.

gtfs_reference works similar to get_gtfs_standards() and I really tried to keep the current structure but I couldn't make it work throughout. Not least because of the new geojson files which necessitate quite some changes in reading and writing files anyway. Also file and field presence were hard coded in get_gtfs_standards() but they were not actually used anywhere in gtfsio. The same goes for enum values which were simply present as additional documented values. So I decided to deprecate get_gtfs_standards() in its current state and move all actual implementation to gtfs_reference.

So now this PR has expanded quite a bit and there are a alot of changes under the hood. There should be no changes in interfacing with gtfsio however (except if you use get_gtfs_standards() directly).

I'll try to adapt tidytransit to this state to see if there's any issues popping up there. I'm not sure how straightforward converting a json list to an sf object actually is.

[polettif]

tidytransit is ready to use the new features. There should be minimal changes to existing gtfsio workflows. Unfortunately I can't check the impact on gtfstools.

Do you see any issues with this @dhersz @rafapereirabr @mpadge?

[mpadge]

Thanks so much for putting in the great work here @polettif !!

[polettif]

@rafapereirabr Do you have any objections to merging this PR?

[rafapereirabr]

This all looks good to me, @polettif ! I think you're good to go and merge the PR. I would be glad to merge the PR, but I'm currently not listed as co-author of the package so I'm not sure I have the 'proper authority' to do so .