Tangodict code improvements

[clairecw]

Implements contains in the Tango*Dictionary classes, replacing any unnecessary calls to .keys().
Also adds tests for TangoDictionary accordingly.

Fixes #171.

[fanpu]

Hi @clairecw , thank you for your PR! It looks really good in general, and I really appreciate the fact that there are unit tests. A couple of more complicated unit tests would also be helpful. Do you have other instructions for end-to-end testing that you could share as well?

[clairecw]

Hi @fanpu, thanks so much for the feedback! Yes, I've added unit tests testing a bit more functionality for the dictionary object specifically - if you could TAL when you get the chance and see if there's anything else you had in mind that'd be awesome!
Lmk also if you'd like to see unit tests for the other Tango objects.

As for E2E tests, I suppose there could be tests for each of the functions in TangoServer, and maybe another file testing the endpoints themselves.

[fanpu]

Great job and thanks! I've looked through the changes and tests and they look good, and I've also tested it with Autolab.

One last change before it's ready to be merged: could you use the unittest methods for asserting the tests instead of the native Python assert? You can refer to this file for examples.

[fanpu]

Tests for other Tango objects would also definitely be greatly appreciated, see #183 for some ideas

Just to clarify, those tests would be better for a separate pr if you are intending to work on them, the pr right now is good apart from the assert changes

[fanpu]

Excellent job and thanks once again for your contributions!