Initialize beam with reasonable default values

and preserve central value while switching between distributions.

When (in scan) central value is not needed to be set, it is not shown and not preserved.

• some cleaning.

Resolves #1030 (closed)

requested review from @j.wuttke

assigned to @m.svechnikov

added 4 commits

• c5f2ccd1 - init m_wavelength_item in SourceItem
• a81e347d - init lambda and angle with default values
• 12a6e852 - preserve central value of distribution
• 675665cc - hide and do not preserve mean in scans

Compare with previous version

approved this merge request

merged

mentioned in commit 91fe2070