Hx711-multi

- Calibration
- Somewhat of a large feature addition to assist with calibration. The HX711 class now includes a function to launch a sequence to help with calibration. This is something that needs to be performed in almost all cases, so adding this functionality should help during the setup process. The calibration sequence prompts the user to confirm known weights and then calculates a weight multiple based on the measurements and those known weights.
- Get Commands
- Due to confusion about how the read_raw() and read_weight() functions work, I added get_raw() and get_weight() to clarify when a new measurement will or will not be taken.
- Bugs
- I found some bugs when the user supplies an integer for the dout_pins (this is a supported feature). This was supposed to allow the user to run hx711-multi with only a single ADC and not force them to use list indexing everywhere, but a bug made a small part of it not work. I fixed that bug and now you can use hx711-multi without the "multi" functionality if you pass an integer for the `dout_pins` input.

No large functional changes. Backward compatible unless you were accessing load cell data within the hx711 class for debugging.

1. Converted LoadCell class name and all references to load cells to "ADC". The term "LoadCell" was unused in the main part of the code, just behind the scenes. But if you were to access that data for debugging purposes, you will need to update to "LoadCell" and "_load_cells"
2. Increased max readings limit from 100 to 10k so that users with 80Hz sampling aren't limited to 1Hz effective sampling

Primary change is to not overwrite 'weight' attribute of load cell before each read. This allows us to track weight even if a single read fails.
Also, we now retry the read when zeroing to make sure we get a good zero reference