Change test_multi_column.py into a unittest.

Cherry-Picked from 84be61a3.
Improve docstrings.
Comment YAML configuration file lt3maps.yaml.
Delay first injection if it starts SEQ.

This way, there is time for the signal to be high before it goes low to
inject.
Add extra control configuration before output.

Configuration sets HITLD and S0 to 0 again (as specified in the FE-I4
manual).
Clean up scan_inject.py.

Remove a print statement, change repeated commands to a for loop.
Fix bug: SEQ writes the wrong length of injection.

Bug was introduced in commit 9655a87354d089de716bc417cada90c02fb078b1.

Fixed by keeping track of "number of bits written to sequence" and
updating that value in the specific case that the first command is an
injection (dealt with in a special way).

Along the way, changed the _block_length member variables to a dict,
_block_lengths[...]. Acceptable keys are currently 'pixel', 'global',
and 'inject'.
Set up to debug injection output.
Fix order of LD_IN0_7.

Should be backwards.
Allow command-line argument to change column #.

Also commented out the pixel sr output.
Update LD_IN0_7 bitarray to be reversed (normal).

Responding to the change from commit 98b6add2.
Merge branch 'feature/injection' of github.com:musicalrunner/atlas_usbpix_control into feature/injection
Update LD_IN0_7 bitarray to be reversed (normal).

Responding to the change from commit 98b6add2.
Correct configurations for injection.

Learned more about latches. Steps:
- Send appropriate values into shift register.
- Open latch with LDENABLE_SEL and LD_IN0_7 bits high
- Close latch same way but with bits low
Merge branch 'feature/injection' of github.com:musicalrunner/atlas_usbpix_control into feature/injection