Issue 2901 parameters csv

[valentinsulzer]

Description

In recent versions of PyBaMM, we recommend using python files to specify parameter sets (as opposed to csv files in early versions). This PR removes all the functionality that was included to make loading from csv files possible. Loading from csv files is no longer possible, and Python files should now always be used instead.

Fixes #2901

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 project coverage change: -0.01 ⚠️

Comparison is base (9d60f05) 99.71% compared to head (25ddb12) 99.70%.

❗ Current head 25ddb12 differs from pull request most recent head 4c1b08f. Consider uploading reports for the commit 4c1b08f to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2959      +/-   ##
===========================================
- Coverage    99.71%   99.70%   -0.01%     
===========================================
  Files          273      245      -28     
  Lines        19032    18628     -404     
===========================================
- Hits         18977    18573     -404     
  Misses          55       55              

Impacted Files	Coverage Δ
pybamm/__init__.py	100.00% <ø> (ø)
pybamm/util.py	100.00% <ø> (ø)
pybamm/parameters/parameter_values.py	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[brosaplanella]

Looks good, thanks! You need to add to the CHANGELOG before merging.