Issue 570 processed var warning

[valentinsulzer]

Description

Add warning if processed variable is called outside range
Fixes #570

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.

• 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 great thanks @tinosulzer !

[rtimms]

looks like the test_logger test needs updating as the default logger level has changed

[codecov]

Codecov Report

Merging #681 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #681      +/-   ##
==========================================
+ Coverage   98.56%   98.56%   +<.01%     
==========================================
  Files         170      170              
  Lines        8517     8521       +4     
==========================================
+ Hits         8395     8399       +4     
  Misses        122      122

Impacted Files	Coverage Δ
pybamm/quick_plot.py	96.75% <ø> (ø)	⬆️
pybamm/processed_variable.py	97.23% <100%> (+0.03%)	⬆️
pybamm/logger.py	100% <100%> (ø)	⬆️

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