Issue 1437 concatenation variable

[valentinsulzer]

Description

Add a ConcatenationVariable which is automatically created by a concatenation of variables and is a bit easier to process

Fixes #1437

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 #1453 (ce83b9e) into develop (c15b894) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1453   +/-   ##
========================================
  Coverage    98.34%   98.34%           
========================================
  Files          281      281           
  Lines        16469    16482   +13     
========================================
+ Hits         16197    16210   +13     
  Misses         272      272           

Impacted Files	Coverage Δ
pybamm/__init__.py	94.91% <100.00%> (-0.05%)	⬇️
pybamm/discretisations/discretisation.py	99.78% <100.00%> (-0.01%)	⬇️
pybamm/expression_tree/binary_operators.py	99.66% <100.00%> (ø)
pybamm/expression_tree/concatenations.py	98.06% <100.00%> (+0.27%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c15b894...ce83b9e. Read the comment docs.

[rtimms]

thanks @tinosulzer !