Include pvarray in __init__.py

[kandersolar]

• Closes #xxxx
• I am familiar with the contributing guidelines
• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

pvlib.pvarray doesn't behave as you might expect:

In [1]: import pvlib

In [2]: pvlib.pvsystem.sapm
Out[2]: <function pvlib.pvsystem.sapm(effective_irradiance, temp_cell, module)>

In [3]: pvlib.pvarray.pvefficiency_adr
Traceback (most recent call last):

  Cell In[3], line 1
    pvlib.pvarray.pvefficiency_adr

AttributeError: module 'pvlib' has no attribute 'pvarray'

This is because it was not listed in the __init__.py file. This one-line PR lists it there so that the behavior becomes:

In [1]: import pvlib

In [2]: pvlib.pvarray.pvefficiency_adr
Out[2]: <function pvlib.pvarray.pvefficiency_adr(effective_irradiance, temp_cell, k_a, k_d, tc_d, k_rs, k_rsh)>