Open-shell doublet as default for uneven number of electrons

[thfroitzheim]

This PR reverts part of the #1235 following discussions with @stefangrimme.
An error occurs only if the number of unpaired electrons specified via the .UHF file or the --uhf flag is inconsistent with the total number of electrons (i.e., it is even and the number of electrons is odd or the other way around).
In all other cases, we specify a default:

• closed-shell singlet (unpaired=0) if the number of electrons is even
• open-shell doublet (unpaired=1) if the number of electrons is odd

Given that we use the first default all of the time, the low-spin open-shell default is the logical extension.

[Copilot]

Pull request overview

This PR adjusts the default spin handling so odd-electron systems can default to an open-shell doublet instead of always failing multiplicity consistency checks, and updates user-facing documentation/output to reflect the effective unpaired-electron count.

Changes:

• Defaults odd-electron xTB calculations with no nonzero UHF assignment to nopen = 1.
• Prints wfn%nopen instead of mol%uhf in SCF setup output.
• Updates CLI help and man page text for the --uhf default.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/prog/main.F90	Updates wavefunction spin/default consistency logic.
src/scf_module.F90	Reports the initialized wavefunction unpaired-electron count.
src/xhelp.f90	Documents parity-dependent default --uhf behavior.
man/xtb.1.adoc	Updates man page documentation for --uhf defaults.

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