Add a WeightedCost class

[NicolaCourtier]

Feature description

Multi-objective optimisation via the Cost class.

Motivation

To allow the summing of multiple cost functions for multi-objective optimisation and to show how the base objects may be combined.

Possible implementation

Add a WeightedCost subclass of BaseCost that combines a list of existing Cost classes, with optional weighting, into a single Cost object to be passed to the optimiser as usual.

Additional context

No response