remove external_variables

[TomTranter]

Description

Remove external_variables which have functionality that is now completely replicable with input parameters. Motivation is to simplify code and reduce redundant functionality. Also relates to a liionpack issue with handling the two types of data seperately and is cleaner to use inputs.

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

[codecov]

Codecov Report

Base: 99.72% // Head: 99.73% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (bff1f46) compared to base (97e2785).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #2502    +/-   ##
=========================================
  Coverage    99.72%   99.73%            
=========================================
  Files          270      270            
  Lines        19487    19360   -127     
=========================================
- Hits         19434    19308   -126     
+ Misses          53       52     -1     

Impacted Files	Coverage Δ
pybamm/expression_tree/exceptions.py	100.00% <ø> (ø)
...mm/expression_tree/operations/convert_to_casadi.py	100.00% <ø> (ø)
pybamm/expression_tree/variable.py	100.00% <ø> (ø)
...full_battery_models/equivalent_circuit/thevenin.py	100.00% <ø> (ø)
pybamm/models/submodels/base_submodel.py	100.00% <ø> (ø)
pybamm/parameters/parameter_values.py	100.00% <ø> (ø)
pybamm/solvers/casadi_solver.py	99.66% <ø> (ø)
pybamm/solvers/idaklu_solver.py	99.06% <ø> (ø)
pybamm/spatial_methods/finite_volume.py	98.14% <ø> (-0.02%)	⬇️
pybamm/spatial_methods/spatial_method.py	96.29% <ø> (-0.07%)	⬇️
... and 7 more

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[valentinsulzer]

Existing changes look good but there are more lines to remove, see coverage

[TomTranter]

@tinosulzer tests passing but some benchmarks are failing. How can I run just the failing ones locally easily?

[valentinsulzer]

Thanks @TomTranter