Fix starting_solution issues

[brosaplanella]

Description

This PR is to solve the issue with the starting_solution when the final cycle was not saved. As discussed in #3101, I think the most robust way forward is to always save the final cycle (even at the expense of consuming a bit more of memory), as it also fixes the last_state.

It also fixes another bug (described in the same issue) which caused the simulation to always take the initial_start_time from the experiment, but it should be taken from the starting_solution if it exists.

Fixes #3101

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 (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
• The documentation builds: $ python run-tests.py --doctest

You can run unit and doctests together at once, using $ python run-tests.py --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

Patch coverage: 100.00% and no project coverage change.

Comparison is base (f4dfbe7) 99.71% compared to head (9efd58b) 99.71%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3177   +/-   ##
========================================
  Coverage    99.71%   99.71%           
========================================
  Files          248      248           
  Lines        18871    18900   +29     
========================================
+ Hits         18817    18846   +29     
  Misses          54       54           

Files Changed	Coverage Δ
pybamm/simulation.py	100.00% <100.00%> (ø)
pybamm/solvers/solution.py	100.00% <100.00%> (ø)

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