General code cleanup

[BjarneBitscrambler]

Filenames should be standardized.
Consider removing SPI files and just leaving stubs that could be used later if desired.
Consider consolidating the user-edited files into one.
Read up on what's required to make an Arduino library.
Decide whether to convert to C++ or leave as (mostly) C.
Define an interface to outside world - one that will be compatible with SensESP.

[BjarneBitscrambler]

Regarding the above tasks:

• Filenames should be standardized.
• Consider removing SPI files and just leaving stubs that could be used later if desired.
• [no] Consider consolidating the user-edited files into one. Decided to keep functions grouped by purpose.
• [no] Read up on what's required to make an Arduino library. Will open separate issue later.
• [no] Decide whether to convert to C++ or leave as (mostly) C. Decided to leave as C.
• Define an interface to outside world - one that will be compatible with SensESP. Will modify the interfaces in control_output.*

Will close this issue now. Code functions. Will plan to do future refactoring as needed. The current file structure will be documented in a /docs subfolder.