Add doctests everywhere and include tests in Travis CI builds

[athityakumar]

Is your feature request related to a problem? Please describe.
It'd be good to check if docstring documentation is present for all methods/modules.

Describe the solution you'd like
The one who takes up this issue should:

• Add docstring documentation wherever it's not present
• Include it in Travis CI builds like pytest --doctest-modules ... in .travis.yml

Describe alternatives you've considered

NA

Additional context

NA