Added convertion from tthrad to RecD2_NM

[vallsv]

Here is an update of the helper convertor to handle tth_rad to d^-2.

It's a rework of the info from pyFAI.units, but i would be happy to have a double check here.
Could you please check if the math is right?

BTW: I also have that code in Flint for now, to be sure it is working with previous pyFAI versions.

[kif]

The math do look OK. Was this feature requested by MX beamlines ?

[kif]

Just checking what's going wrong with RTD. Your PR is OK.

[vallsv]

Thanks for the check

The math do look OK. Was this feature requested by MX beamlines ?

ID15, to display the value in d^-2 in a tooltip

[vallsv]

In fact they prefer to display the stuff in d*2_A^-2, there is a registration unit for that, but there is no global named var.

Do you think it is fine if i add the following in the units module?

RecD2_A = RADIAL_UNITS["d*2_A^-2"]

[vallsv]

And the equation would be the same as?

        q_A = (4.e-10 * numpy.pi / wavelength) * numpy.sin(.5 * twoTheta)
        return (q_A / (2.0 * numpy.pi)) ** 2

[kif]

The scale factor differs by a factor 100 (i.e. 10²) that's all.

[kif]

In this case I would rather do smth like this:

elif unit.space == "d*2":
    q_A = (4.e-9 * numpy.pi / wavelength) * numpy.sin(.5 * twoTheta)
    rec_d2_nm =  (q_A / (2.0 * numpy.pi)) ** 2
    return rec_d2_nm/unit.scale

[kif]

The same could be done to simplify the handling of q, 2th and r, each time, just divide by the scale.