make variables_and_events attribute and remove some events

[valentinsulzer]

Description

Some changes to help debugging events

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

Merging #2158 (361cc79) into develop (6877eac) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #2158   +/-   ##
========================================
  Coverage    99.38%   99.38%           
========================================
  Files          356      356           
  Lines        19297    19309   +12     
========================================
+ Hits         19178    19190   +12     
  Misses         119      119           

Impacted Files	Coverage Δ
pybamm/models/base_model.py	99.27% <100.00%> (+0.01%)	⬆️
...ls/electrolyte_diffusion/constant_concentration.py	100.00% <100.00%> (ø)
...amm/models/submodels/porosity/constant_porosity.py	100.00% <100.00%> (ø)
pybamm/solvers/solution.py	100.00% <100.00%> (ø)

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