change: get dimensions for aq.ArrayVar automatically

[Yash-10]

Issue #13:

Description of changes:

• In the definition of ArrayVar in types.py, the dimensions keyword argument is no longer required but inferred from the initial expression. The initial expression, however, becomes a necessary argument following the comments made on the issue.
• args[0] is used because _ClassicalVar in oqpy's ArrayVar suggests init_expression is the first argument.

Testing done: Yes.

Merge Checklist

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General

• [X] I have read the CONTRIBUTING doc
• [X] I used the commit message format described in CONTRIBUTING
• [ ] I have updated any necessary documentation, including READMEs and API docs (if appropriate)
  • Perhaps not required since the high-level interface only contains args and kwargs, which remain unchanged?

Tests

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[codecov[bot]]

Codecov Report

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

Project coverage is 100.00%. Comparing base (ecce2bc) to head (1b85446). Report is 15 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #22 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 44 43 -1 Lines 1936 1903 -33 Branches 319 320 +1 ========================================= - Hits 1936 1903 -33 ```

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