Issue 2760 step

[valentinsulzer]

Description

Adds offset to the start of steps which follow from previous solution. This avoid duplicate time in solution and ensures that concatenations of steps (/cycles) have the same length as the whole cycle (/solution)

Fixes #2760

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

[brosaplanella]

Looks good, thanks! Happy to merge once tests pass

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (460e684) 99.68% compared to head (7e2e0ed) 99.68%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2773   +/-   ##
========================================
  Coverage    99.68%   99.68%           
========================================
  Files          272      272           
  Lines        19017    19024    +7     
========================================
+ Hits         18958    18965    +7     
  Misses          59       59           

Impacted Files	Coverage Δ
...s/full_battery_models/lithium_ion/electrode_soh.py	100.00% <100.00%> (ø)
pybamm/settings.py	100.00% <100.00%> (ø)
pybamm/solvers/base_solver.py	100.00% <100.00%> (ø)

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