For noiseless QVMs don't measure all qubits

[notmgsk]

Description

QuantumComputer.run_and_measure inserts MEASURE instructions for all qubits into the program before it is sent for execution by the QPU or QVM. When the target is the QVM this may lead to memory exhaustion: the QVM allocates enough memory to track a wavefunction for the number of qubits used in the program. For a larger number of qubits (around 32), one will use over 1GB of memory which causes the QVM to (rather ungraciously) crash. This PR fixes that by measuring only the qubits used in the program if and only if the target is the QVM and there is no associated noise model.

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.
• (New Feature) The docs have been updated accordingly.
• (Bugfix) The associated issue is referenced above using auto-close keywords.
• The changelog is updated, including author and PR number (@username, gh-xxx).

[kalzoo]

Thanks! Users will appreciate this.

[kalzoo]

lgtm, pending a small typo. Thanks!