#1785 override sympy operators exp and atan

[brosaplanella]

Description

Overrode the sympy operator for Exponential and Arctan. I think the only left are Max and Min but not sure if we want to override them by the Max and Min from sympy.

Fixes #1785

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

[priyanshuone6]

Could you add some tests

[codecov]

Codecov Report

Merging #1786 (e7d1005) into develop (77f7169) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1786   +/-   ##
========================================
  Coverage    99.28%   99.28%           
========================================
  Files          343      343           
  Lines        18911    18998   +87     
========================================
+ Hits         18776    18863   +87     
  Misses         135      135           

Impacted Files	Coverage Δ
pybamm/expression_tree/functions.py	100.00% <100.00%> (ø)
pybamm/solvers/solution.py	100.00% <0.00%> (ø)
pybamm/simulation.py	98.06% <0.00%> (+<0.01%)	⬆️

---

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 77f7169...e7d1005. Read the comment docs.