add tensors and basic operators

[aabills]

Description

Adds tensors and some basic operators. Useful for some models which need these types of things.

Fixes # (issue)

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #)

Important checks:

Please confirm the following before marking the PR as ready for review:

• No style issues: nox -s pre-commit
• All tests pass: nox -s tests
• The documentation builds: nox -s doctests
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

❌ Patch coverage is 90.44586% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.59%. Comparing base (3a25c09) to head (edf1919).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/pybamm/expression_tree/tensor_field.py	90.27%	7 Missing ⚠️
src/pybamm/expression_tree/binary_operators.py	84.00%	4 Missing ⚠️
src/pybamm/spatial_methods/finite_volume_2d.py	92.10%	3 Missing ⚠️
src/pybamm/expression_tree/symbol.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5335      +/-   ##
===========================================
- Coverage    98.64%   98.59%   -0.05%     
===========================================
  Files          323      324       +1     
  Lines        28063    28211     +148     
===========================================
+ Hits         27682    27816     +134     
- Misses         381      395      +14     

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[valentinsulzer]

would left and right ever be constant (i.e. use simplify_if_constant) ?