Power mode bug

[valentinsulzer]

Description

Fix a bug in is_matrix_one that was causing an incorrect result when using power mode (and probably also voltage mode).

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

[valentinsulzer]

Could probably do with adding more tests to catch this kind of bug earlier

[codecov]

Codecov Report

Merging #1735 (1a72c32) into develop (0160810) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1735   +/-   ##
========================================
  Coverage    99.28%   99.28%           
========================================
  Files          343      343           
  Lines        18864    18864           
========================================
  Hits         18729    18729           
  Misses         135      135           

Impacted Files	Coverage Δ
pybamm/spatial_methods/finite_volume.py	98.15% <ø> (ø)
pybamm/expression_tree/symbol.py	98.98% <100.00%> (ø)

---

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 0160810...1a72c32. Read the comment docs.