FinalTime and WeightedDiscreteTimeSum nodes

[martinjrobins]

Description

We have a couple of expression tree nodes that perform a sum/integral over time:

• DiscreteTimeSum
• ExplicitTimeIntegral

Some other useful related nodes are:

• FinalTime: returns the value of a single child node at the final time point
• WeightedDiscreteTimeSum: like DiscreteTimeSum but provides a weighting for each time point

Motivation

These are used to implement loss functions that are used in optimisation and param inference libraries like pybop

Possible Implementation

The ProcessedVariableTimeIntegral implements the functionality of DiscreteTimeSum and ExplicitTimeIntegral. It should be straightforward to add these two new nodes into this class

Additional context

No response