#1329 reformat Solution to avoid concatenating y

[valentinsulzer]

Description

Reformat Solution to avoid concatenating y (for performance)

Fixes #1329

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 #1331 (2542734) into develop (6e1aa13) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1331   +/-   ##
========================================
  Coverage    98.12%   98.13%           
========================================
  Files          272      272           
  Lines        15443    15457   +14     
========================================
+ Hits         15154    15169   +15     
+ Misses         289      288    -1     

Impacted Files	Coverage Δ
pybamm/parameters/parameter_values.py	99.35% <ø> (ø)
pybamm/plotting/quick_plot.py	100.00% <ø> (ø)
pybamm/simulation.py	97.07% <100.00%> (+0.15%)	⬆️
pybamm/solvers/algebraic_solver.py	100.00% <100.00%> (ø)
pybamm/solvers/base_solver.py	99.12% <100.00%> (-0.03%)	⬇️
pybamm/solvers/casadi_algebraic_solver.py	98.71% <100.00%> (ø)
pybamm/solvers/casadi_solver.py	99.54% <100.00%> (ø)
pybamm/solvers/dummy_solver.py	100.00% <100.00%> (ø)
pybamm/solvers/idaklu_solver.py	90.35% <100.00%> (+0.08%)	⬆️
pybamm/solvers/jax_solver.py	98.30% <100.00%> (ø)
... and 6 more

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[rtimms]

neat solution! just a few comments, thanks @tinosulzer !

[brosaplanella]

Looks good to me, thanks Tino!