fix sim.t_eval when timescale is function of input

[TomTranter]

Description

Use evaluated timescale from solution when doing sim solve

Fixes #1217

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: $ flake8
• All tests pass: $ python run-tests.py --unit
• The documentation builds: $ cd docs and then $ make clean; make html

You can run all three 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

[rtimms]

thanks @TomTranter , don't forget to update the changelog

[codecov]

Codecov Report

Merging #1218 into develop will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1218   +/-   ##
========================================
  Coverage    97.92%   97.92%           
========================================
  Files          249      249           
  Lines        14146    14146           
========================================
  Hits         13852    13852           
  Misses         294      294           

Impacted Files	Coverage Δ
pybamm/simulation.py	96.81% <100.00%> (ø)

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[TomTranter]

Didn't realize we had a changelog!