Affine-parametric Operators, Parametric ROMs

[shanemcq18]

Support for affine-parametric problems:

• Affine-parametric operator classes AffineConstantOperator, AffineLinearOperator, etc.
• Parametric model classes ParametricContinuousModel, ParametricDiscreteModel.
• ParametricROM class.
• Updates to operator / model documentation.

Renamed interpolatory operators / model classes from Interpolated<Name> to Interp<Name>.
Old names are deprecated but not yet removed.

Miscellaneous:

• Reorganized and expanded tutorials.
• Added and documented opinf.utils.TimedBlock context manager for quick timing of code blocks.
• Updated structure for some unit tests.
• Refactored interpolatory operators.
• Standardized string representations, added [Parametric]ROM.__str__().
• Removed some public functions from operators, regrouped in operators._utils.
• Removed some public functions from models, regrouped in models._utils.