Delta residual fix

[igresh]

Delta was not being treated as an angle when residuals were being calculated. For instance, the difference between 2˚ and 358 ˚ was calculated at 356˚, rather than 4˚. This typically doesn't cause an issue when datasets do not contain Delta close to 0/360 (so avoided detection) but can cause serious issues.

The current pull request fixes this calculation using:

(delta_model - delta_data + 180) % 360 – 180

Which is an efficent way of calculating the circular distance between two angles.

[andyfaff]

I don't think it's quite right, because it can furnish negative numbers:

def cd2(angle1, angle2):
    return (angle1 - angle2 + 180) % 360 - 180

cd2(358, 2) # -4

I might put in the circular distance I sent you. It's great that you added the test.

[igresh]

Is that an issue given that the standard way of calculating residuals (i.e., model - data) can also furnish negative numbers? Won't the result always be squared anyway?

[andyfaff]

In this case I don't think there are modes of analysis where the sign matters.

Also, what about psi?

[igresh]

Psi is the ratio of s and p polarised light, so isn't an angle. I think the way we calculate psi residuals are ok?

[andyfaff]

Ok, let's leave it there then.

[igresh]

Also the docstring specified degrees in radians, but delta is always supplied in degrees (in refellips). I don't think it matters for the logic as long as the period is also in degrees, but might cause confusion down the line.

[igresh]

I'm happy. Lets merge?

[andyfaff]

What other changes did you want to make?

[igresh]

I've got a few file readers (filmsense and Woolam M2000) + tests.