Rename TomographyExperiment to Experiment

[karalekas]

Description

I prefer the shorter name, and these hybrid/tomography-like experiments are the focus of our quantum cloud platform.

Checklist

• The above description motivates these changes.
• There is a unit test that covers these changes.
• All new and existing tests pass locally and on Travis CI.
• Parameters and return values have type hints with PEP 484 syntax.
• Functions and classes have useful Sphinx-style docstrings.
• All code follows Black style and obeys flake8 conventions.
• The changelog is updated, including author and PR number (@username, gh-xxx).

[appleby]

I think TomographyExperiment is more descriptive (and easier to grep for), but also it's nice that the pyquil.experiment module exports a thing called Experiment and the symmetry with ExperimentSetting is nice.