RLS11 — Multiplicity of Architecture::WeightedModel

In Figure 7.6 - RoLo Error, multiplicity of Architecture::WeightedModel for Architecture::LinearMixtureModel is defined as "1..". However, as there may be cases that no model exists, it is better to define it as "0..".

Concerning also issue 16189 where the multiplicity of weight attribute in
Architecture::WeightedModel is raised, we perform the following modifications:
1) Define a new Architecture::Model class which is derived from
Architecture::PositionElement. Although no attributes or methods added here,
this class is for representing probability distributions or models, with a position
in the space and error distribution.
2) Modify Architecture::WeightedModel class to be derived from
Architecture::Model class and remove the posElem attribute (this is not
necessary any more due to its parent class-). Thus, only models with weights
are represented by the WeightedModel class and thus the ‘weight’ attribute is
left as mandatory.
3) Modify Architecture::LinearMixtureModel class to aggregate
Architecture::Model instances instead of Architecture::WeightedModel, so that
when the weight in the models shall not be specified. the Model class
instance shall be used. At the same time, modify the multiplicity of the
‘models’ attribute as “0..*.” Also, add some description in the definition of
LinearMixtureModel class to indicate how the aggregation of non-weighted
models shall be interpreted.