#1066 add numpy function sqrt, sin, cos and exp to convert_to_casadi

[martinjrobins]

Description

Fixes #1066

Type of change

• 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

[codecov]

Codecov Report

Merging #1067 into develop will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1067   +/-   ##
========================================
  Coverage    97.72%   97.72%           
========================================
  Files          237      237           
  Lines        12413    12435   +22     
========================================
+ Hits         12130    12152   +22     
  Misses         283      283           

Impacted Files	Coverage Δ
pybamm/expression_tree/array.py	100.00% <100.00%> (ø)
pybamm/expression_tree/interpolant.py	100.00% <100.00%> (ø)
...mm/expression_tree/operations/convert_to_casadi.py	94.79% <100.00%> (+1.54%)	⬆️
pybamm/expression_tree/symbol.py	97.44% <100.00%> (ø)

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[valentinsulzer]

Thanks @martinjrobins ! Merging