silx.io.spech5: Fixed unit cell header line interpretation

[t20100]

This PR fixes the header line that is interpreted as unit_cell, it was #G1 and is now #G2.
This looked like a bug, but if someone with good knowledge of SPECfile can check, that'd great.

Related to:

• silx.io.spech5: Make SpecH5 resilient to malformed #G1 and #G3 header lines #3507 (comment)
• error reading spec file with silx.io.open() #3506 (comment)

[t20100]

attn @newville @maurov

[vallsv]

Any feed back here?

[maurov]

Hello, according to the Spec manual, #G2 is the field for the lattice constants. Personally, I would merge like this.

[t20100]

In another documentation, the scans one, you can find:

#G1
Specifies parameters associated with the diffractometer geometry. The parameters are those from the U[] array, which include lattice constants and orientation reflections.

We've checked what the macro does, and it saves the U[] array as #G1 and there is no #G2.

BTW, support of unit_cell as #G1 was introduced in #722 and it is like that since 4 and a half years.

[maurov]

@t20100 I have no strong opinion on this, as I have never used #G* parameters myself. Maybe @vasole knows some beamlines using them.

[t20100]

I have no strong opinion on this,
@maurov me neither!

Documentation is inconsistent, but the code of the SPEC macro that BCU just checked is clear: it is using #G1, so I'm closing this.

[newville]

@t20100 @maurov Thanks!