feat: add discrete time sum expression tree node

[martinjrobins]

Description

Adds two new subclasses to pybamm.Symbol:

1. pybamm.DiscreteTimeData which is an interpolant for holding time series data
2. pybamm.DiscreteTimeSum which is a unary operator representing a sum of its child evaluation over a set of data time points. Its child must have exactly one pybamm.DiscreteTimeData object.

When pybamm.DiscreteTimeSum is evaluated normally it returns the evaluation of its child. If it is evaluated as part of a pybamm.Solution then it interpolates this solution onto the data time points and sums these over the time axis

This also updates the evaluation of pybamm.ExplicitTimeIntegral. Previously this performed a trapezoidal integral over all the time points passed to the __call__ function of pybamm.ProcessedVariable. Now it performs the integral over the solution time points, which is more consistent with its definition and always returns the same value irrespective of the time values that the user passes to __call__. In the future this integral should be updated to use the gradient information of the solution if available.

Fixes #4485

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)

Key checklist:

• No style issues: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python run-tests.py --all (or $ nox -s tests)
• The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ nox -s quick).

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.42%. Comparing base (9e62b66) to head (9b3d13f).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4501   +/-   ##
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  Coverage    99.42%   99.42%           
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  Files          297      299    +2     
  Lines        22685    22739   +54     
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+ Hits         22554    22608   +54     
  Misses         131      131           

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[MarcBerliner]

Looks great! Thanks for also separating out and simplifying the ExplicitTimeIntegral. The changelog needs updating as well.