cubicweb-processing #2409933 ParameterValue value should not be mandatory when linked to an ObjectiveDefinition [validation pending]

This would be nicer and avoid bad parameter value interpretations.

We should however ensure a computation cannot be launched until all its parameters values are set (either manually or using a copy of an objective linked to it).

done in0.2.0
load left0.000
closed by#09685c3836c8 ParameterValue value is no more required, but if not supplied, a link to an objective must exist (closes #2409933)