delay xarray.DataArray initialization

[valentinsulzer]

Description

This speeds up simulations by 5x because xarray.DataArray initialization is slow
Before (560ms, 2% time spent solving the model):

After (130ms, 10% time spent solving the model):

Test code (this example is a "worst-case scenario" for overheads where solving the simulation is very quick, it highlights the other parts of the code that are slow):

import pybamm
import matplotlib.pyplot as plt

pybamm.set_logging_level("NOTICE")
experiment = pybamm.Experiment(
    [
        (
            "Discharge at 1C for 1 minute",
            "Rest for 1 minute",
            # "Charge at 5 A until 4.2 V",
            # "Hold at 4.2 V until 10 mA",
            # "Rest for 1 hour",
        ),
    ]
    * 10
)
model = pybamm.lithium_ion.SPM()
parameter_values = pybamm.ParameterValues("Chen2020")

sim = pybamm.Simulation(
    model,
    experiment=experiment,
    parameter_values=parameter_values,
    solver=pybamm.CasadiSolver("fast with events"),
)
sim.solve()

There are still some significant overheads, we should be as close to 100% time spent solving the model as possible, but this is a step in the right direction.

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)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

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 99.60%. Comparing base (ed6df7e) to head (0767c8d).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3862      +/-   ##
===========================================
- Coverage    99.60%   99.60%   -0.01%     
===========================================
  Files          259      259              
  Lines        21284    21272      -12     
===========================================
- Hits         21200    21188      -12     
  Misses          84       84              

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[kratman]

I don't see any issues with this and the tests pass