this eases migration of SessionModel/SessionItem

use it directly instead via DocumentModel

BasicVector: inline, modernize, cleanup

See merge request !480

no unit tests for trivial getters

See merge request !479

reduce SessionModel/SessionItem usage

See merge request !478

...and simplify code

Add and use dedicated methods. This will ease SessionModel migration

simplify RealSpace generation

See merge request !477

Reduce SessionModelView ("Model tech view")

See merge request !476

Custom editors are now only used from the Fit-Parameter-tree. In this tree, only doubles and Combos exist, and doubles use only scientific spin boxes.
All other custom editors are obsolete

...and remove obsolete code.

The "model tech view" is only kept for further migration (to understand the SessionModel). However, it is not allowed anymore to edit the model in this view.

-> SessionModelDelegate
-> FitParameterDelegate

Harmonized and less generic UI for minimizers

See merge request !475