update tria function

[suvanbanerjee]

Changed the function to abs(x % 4 - 2) and span to [0, 4]
@bessman I don't have the required hardware to record the serial traffic.
Closes #218

[suvanbanerjee]

@bessman please confirm is this correct or I have to do something else.

[bessman]

There is some kind of problem with the new implementation. Blue curve is old, orange is new:

I can't immediately see what the problem is. I'll give it a think and get back to you.

[bessman]

Only the span should be changed, not the function. The -1 in the function is necessary to get correct analog offset; pslab-android corrects for this offset in loadTable instead.

Please revert the change to tria, but keep the new span. Replace the recorded test data (tests/recordings/waveform_generator/test_triangle_wave.json) with this updated recording: test_triangle_wave.json

Verify that the test passes (pytest -k test_triangle_wave), then update the PR.

[suvanbanerjee]

@bessman test are working fine

[bessman]

Good work, thanks!