Improved autocomplete for most ehrql things

[rw251]

The type checker/autocomplete now knows:

• the correct type of all column series (e.g. patients.date_of_birth is a DatePatientSeries)
• the return type of almost all methods on the series (e.g. clinical_events.numeric_value.sum_for_patient() is a FloatPatientSeries
• properties on series have the correct type (e.g. patients.date_of_birth.day is a IntPatientSeries)
• the return type for most operators like +, -, >, / etc.

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[evansd]

This is great!

The only part that gave me any pause was the @int_property stuff just because I'm not completely clear on what problem it solves and how it does it. (I'm quite confident there is a problem and that it does solve it, I'd just like to understand it better.)

But it's a small and simple enough bit of code that I'd be happy to merge it now and understand it later.

In terms of testing I'll make a separate ticket to think about options there.

[rw251]

The only part that gave me any pause was the @int_property stuff just because I'm not completely clear on what problem it solves and how it does it. (I'm quite confident there is a problem and that it does solve it, I'd just like to understand it better.)

Having not looked at it since before my break I'm not sure I can remember! It was something to do with how the autocomplete interacted with the @property decorator. I tried a few things, and this seemed to be the simplest. I also considered making it more like a @autocomplete_property[T], but given we currently only have int properties it seemed easier to just do that and not make it more complicated than it needed to be.