feature: Provide access to global qubit register inside AutoQASM program

[rmshaffer]

Issue #, if available: Provide access to global qubit register as aq.qubits from inside an AutoQASM program

Description of changes:

• Add global_qubit_register property to ProgramConversionContext
• Export global qubit register as module-level attribute aq.qubits
  • Note that using aq.qubits requires num_qubits to be specified in aq.main
• Allow user to write loops like for q in aq.qubits:
• Allow user to obtain the number of qubits via len(aq.qubits)
• Implement measure() with no args for measuring all qubits (if num_qubits is specified in aq.main)
• Update example notebooks to use the new syntax where applicable

Testing done:

• Tests added for new functionality
• tox passes, including tox -e notebooks locally

Merge Checklist

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General

• [x] I have read the CONTRIBUTING doc
• [x] I used the commit message format described in CONTRIBUTING
• [x] I have updated any necessary documentation, including READMEs and API docs (if appropriate)

Tests

• [x] I have added tests that prove my fix is effective or that my feature works (if appropriate)
• [x] I have checked that my tests are not configured for a specific region or account (if appropriate)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (ebe3a8b) to head (e8e9a65).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## feature/autoqasm #918 +/- ## ================================================== Coverage 100.00% 100.00% ================================================== Files 170 170 Lines 10452 10478 +26 Branches 2245 2249 +4 ================================================== + Hits 10452 10478 +26 ```

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