[wip] add mpu6050 sensor

[feilongfl]

Fixes #<GitHub-issue-number>.

Make sure all boxes are checked (add x inside the brackets) when you submit your contribution, remove this sentence before doing so.

• This PR is for the dev branch rather than for master.
• This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• I have thoroughly tested my contribution.
• The code changes are reflected in the documentation at docs/*.

<Description of and rationale behind this PR>

[nwf]

Given that this module appears to be only I2C and needs no sophisticated timing handling &c, it may be better off as a Lua module than a C one.

[feilongfl]

@nwf
This sensor requires very frequent read and write operations, and requires a lot of calculations when parsing data(todo). I think that making a c language module will have better performance.

[HHHartmann]

Just for reference: there was also #2548

[devsaurus]

Build fails now since #2838 was merged:

CC app/modules/mpu6050.c
mpu6050.c:10:22: fatal error: c_stdlib.h: No such file or directory
 #include "c_stdlib.h"
                      ^
compilation terminated.

You need to rebase onto current dev and adapt.

[vsky279]

I've missed this PR. I have a Lua version of the MPU9250 driver - see vsky279@232f61d
MPU9250 is actually MPU6050 + AK8975 magnetometer + BMP280.

But I could not make it work reasonably so I dit not file any PR. It was not working even on Arduino with existing libraries.

@feilongfl if you can try this Lua driver?

You are right that there are quite complex calculations to be performed. Maybe C library just handling complex calculations would be the right way to implement this. I wanted to do it in a second step. But I froze on the first step :-).

We've taken similar approach for the new BME280 driver (https://nodemcu.readthedocs.io/en/dev/lua-modules/bme280/) where calculations are in the C module bme280_math (https://nodemcu.readthedocs.io/en/dev/modules/bme280_math/).