Add the ability to use a "dummy load"

[Elliot-Ford]

Problem

The majority of us are working remotely right now and our work currently relies on physical hardware for testing aspects of this driver (like lsl).

Proposal

Add the ability to have a "dummy load", allowing the program to run reading data from a specified file instead of the values from the chip.
I propose this should be a two part feature:

1. Add a flag to the command line arguments (maybe -i) that specifies the data file to be parsed and a function that acts nearly the same as spi_getadc so that we can make a quick conditional check of which source to read from.
2. Using macros make a Makefile compile target that disables the spi and related libraries so a dummy driver can be built on personal machines that might not support such libraries. I suggest using the macro name DUMMY_LOAD_ONLY.