Issue 914 dummy solver

[valentinsulzer]

Description

Add dummy solver for empty models

Fixes #914

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

[rtimms]

looks good, thanks!

[codecov]

Codecov Report

Merging #915 into develop will increase coverage by 0.00%.
The diff coverage is 98.78%.

@@           Coverage Diff            @@
##           develop     #915   +/-   ##
========================================
  Coverage    98.00%   98.00%           
========================================
  Files          207      208    +1     
  Lines        10914    10929   +15     
========================================
+ Hits         10696    10711   +15     
  Misses         218      218           

Impacted Files	Coverage Δ
pybamm/models/event.py	90.47% <50.00%> (ø)
pybamm/solvers/scikits_ode_solver.py	87.32% <88.88%> (+0.55%)	⬆️
pybamm/discretisations/discretisation.py	99.76% <100.00%> (+<0.01%)	⬆️
pybamm/expression_tree/array.py	100.00% <100.00%> (ø)
pybamm/expression_tree/binary_operators.py	94.03% <100.00%> (ø)
pybamm/expression_tree/concatenations.py	97.98% <100.00%> (ø)
pybamm/expression_tree/functions.py	100.00% <100.00%> (ø)
pybamm/expression_tree/independent_variable.py	100.00% <100.00%> (ø)
pybamm/expression_tree/input_parameter.py	100.00% <100.00%> (ø)
...mm/expression_tree/operations/convert_to_casadi.py	93.24% <100.00%> (ø)
... and 17 more

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