Add analog gpio mode

[russellmcc]

This is necessary when you're using a DAC.

[strom-und-spiele]

This is also needed for ADC, and I did (partly verbatim :) ) the same implementation:
https://github.com/strom-und-spiele/stm32f3xx-hal/blob/adc/src/gpio.rs
would suggest to also add pupdr to into_analog.
As stated in table 72 in the reference manual, only 0b00 (floating) is a valid
config if the pin is set to analog mode.

[russellmcc]

Thanks, you are of course correct. I've added pupdr configuration.

[dfrankland]

@strom-und-spiele, @russellmcc, I think y'all might be even better suited than myself to help run this repo. Would either of you be interested in being added as collaborators?

[strom-und-spiele]

Thanks for the praise - I'm actually interested in being able to create PRs, but I'm still deep in the "aah - so this is how that works" stadium ;)

[russellmcc]

Mostly I'm hesitant due to plenty of other commitments for my time, but if you don't mind a somewhat lackadaisical collaborator I'd be happy to help out - I just can't promise I'll be able to devote any significant time to it.

[dfrankland]

@russellmcc I am looking for folks who know more about embedded development, since I am mostly a beginner myself. No commitments needed, just a review here and there. Although, you don't need to be a collaborator to review code 😁