#1105 use approximated step size for D and c calculation

[martinjrobins]

Description

Fixes #1105

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.

• 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

[codecov]

Codecov Report

Merging #1106 into develop will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1106   +/-   ##
========================================
  Coverage    97.81%   97.81%           
========================================
  Files          245      245           
  Lines        13179    13179           
========================================
  Hits         12891    12891           
  Misses         288      288           

Impacted Files	Coverage Δ
pybamm/solvers/jax_bdf_solver.py	99.39% <100.00%> (ø)

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