[Feature] Adding digital noise

[chMoussa]

Closes #33 by:

• Adding a NoiseInstance with different NoiseType values from where we can extract the krauss operators.
• Adding a noise argument to gates as a tuple of NoiseInstance
• Extending apply functions to handle noise and input State provided as a density matrix.
• Extending the api functions to work with density matrices.
• Adding tests
• Adding section noise in docs
• Removing circuit methods sample and expectation as they were redundant with the api's

[RolandMacDoland]

Thanks a lot @chMoussa. Some comments from me but mainly about renaming. Happy to quickly approve after you've addressed them.

[RolandMacDoland]

Thanks a lot @chMoussa. Final and minor comments otherwise LGTM.