Consistent terminology in tasks names

[GiudGiud]

Motivation

To "create" an object we name tasks like:

• add_function
• setup_executioner / setup_predictor
• create_problem

in Moose.C

it's not great because one does not know if there is a previous action doing the original creation before the 'setup' phase

Design

Consistent choice in verbs. Probably add_ for everything that is plural, and create_ for everything that is unique or add_ for everything

Impact

Easier to develop in moose inner layer