Add GQL documentation

[mabijkerk]

This PR adds documentation for GQL. It partly retrieves the information from this outdated page and partly directly from the ETEngine functions. Some documentation is new.

There are four critical points for review:

• [ ] Are the guidelines under "Writing gqueries" clear, exhaustive etc. or does more information have to be added?
• [ ] Is the guideline for nomenclature of gqueries useful, clear etc.?
• [ ] Are GQL functions missing or should some be removed?
• [ ] Is the categorization of GQL functions clear or should be restructured/renamed (core functions, math functions etc.)?

There are still some GQL functions that need documentation and now have "TO-DO". What was your intention with these functions @kaskranenburgQ?

[kaskranenburgQ]

There are still some GQL functions that need documentation and now have "TO-DO". What was your intention with these functions

@mabijkerk I'm not experienced enough to determine wether they should be included in the documentation and/or how to explain the working of these functions (all fever queries for example). Would be great if you could take a look at these.

[Charlottevm]

First of all, it is really awesome that this knowledge is now available for everyone, absolutely love the fact I can fall back on this documentation for those sparse moments in which I have to write queries. The lay-out (explanation + example) makes it really practical as well. Thank you for that!

Hereby some very small comments:

1. “Gqueries indent first with 4 spaces, then with 2 spaces.” I do not completely understand this comment, could you add one sentence to this explaining why this is relevant? Does the query brake if you do not adhere to this?
2. “MAX(*values)” I noticed the “star” sign in some titles, what does this mean?
3. “to-do” do you intend to work these out further or should they be removed? fine by me to keep it if time does not allow you to work on this anymore. I would say something like ‘documentation still needs to be written, contact quintel if you need further clarification on this topic.
4. “QUERY_FUTURE(key)” The example also showcases ‘2019’ (copied from the one above), since it is for the future it would be nice to note ‘2050’ instead of ‘2019’ (if I understand this query correctly)
5. “a new curve of length length is created,” —> Double lenght
6. I do not understand the “SMOOTH_CURVE(curve, window_size)” & “UPDATE_ABSOLUTE()” yet, if you could provide just a few extra sentences of explanation I think this would be resolved.

Again, good busy!

[kaskranenburgQ]

Thanks for the review @Charlottevm! I have implemented your feedback for the most part. Could you check if I have missed anything? The following functions in the file miss proper documentation and are marked as TO-DO:

FEVER_PRODUCTION_CURVE_FOR_COUPLE FEVER_DEMAND_CURVE_FOR_PEOPLE

@mabijkerk Could you perhaps take a look at these?