Character_LCD_I2C: add support for PCF8574 I2C expander backpack

[Roman3349]

No description provided.

[dhalbert]

Thank you for submitting this! It fixes a long-requested need, e.g. #81.

I have some higher-level comments about how the code is structured:

The PCF8574 support uses the superclasses _write8() support, which sets the bits one at a time. In #80, I changed the code to write the bits four at time, which sped up things considerably. It would be good to do that here too. Unfortunately, the 8-bit-wide GPIO support for the two libraries is not the same. However, small wrapper classes could be written to make an common API. Then an instance of either wrapper class could be passed into the constructor, instead of passing in an instance of one of the library objects.

In addition, this code imports both the PCF8574 and MCP23008 libraries. On boards with limited RAM (e.g. SAMD21), this will use up noticeably more RAM. So passing in a wrapper class instance will avoid needing to import both libraries at once.

An example of this wrapper technique is in https://github.com/dhalbert/CircuitPython_LCD. The internal API presented by that library is not the same, so you wouldn't copy the code directly, but the technique is what I mentioned.

Would you be willing to do this? This would allow other I/O expanders in the future to be added in a modular way (not that I can think of any right now). Thanks.

[Roman3349]

@dhalbert Thank you for your code review.

I fixed the code so that when using PCF8574 the bits are written in bulk similar to MCP23008.

When I have some free time, I'll try to develop a system of wrappers that will allow to extend the library with possible additional I2C expanders and possible different GPIO pin mappings when the used I2C backpack is wired differently.