Add autolux

[mareuter]

This is an implementation of #20 using the contributions from there as well as a couple of ideas from the C++ library. Please make sure the other contributors get credit for this as I just did the "last mile" part. I also corrected a couple of doc strings that were referencing a different library.

[mschlenstedt]

Thanks for implementing this feature! I looked at the code and the datasheet "Designing the VEML7700 Into an Application" (https://www.vishay.com/docs/84323/designingveml7700.pdf) just to understand what you have done here.

I think there might be two mistakes in the code:

1. issue:

According to the datasheet the correction formula should be used for lux > 1000 (page 5 "For illuminations > 1000 lx a correction formula needs to be applied.". But you use the formular when Ambient Light Level > 100. Should here the lux value pre-calculated and then checked for "if lux > 1000"?

2. issue:

The used formular in line 246 is wrong in my opinion:

((6.0135e-13 * lux - 9.3924e-9) * lux + 8.1488e-5) * lux + 1.0023 ) * lux

The power values are missing and the brackets are wrongly set. The original furmula from the datahseet (page 5):

Correcting the lux value with the help of the correction formula yields:
a x (1500 lux)^4 + b x (1500 lux)^3 + c x (1500 lux)^2 + d x (1500 lux) = 1658 lux
With the polynomial coefficients:
a = 6.0135-13
b = -9.3924-9
c = 8.1488-5
d = 1.0023

This is in python:

(6.0135e-13 * lux ** 4) + (-9.3924e-9 * lux ** 3) + (8.1488e-5 * lux ** 2) + (1.0023 * lux)

If you test the correct formular with 1500 lx, you get 1658 lx.

[mareuter]

For Issue 1

So, it's unclear from the flow chart on page 21 which is where the code derives the flow from where that 1000 lux limit on page 5 comes into play. It's not mentioned there. Not sure what to do about that. Any suggestions?

For Issue 2

If I run the formula as is through python, I get your answer:

lux = 1500
(((6.0135e-13 * lux - 9.3924e-9) * lux + 8.1488e-5) * lux + 1.0023 ) * lux
1658.1429843

The code formatting makes it look a bit weird by moving the opening parentheses to the line above.