Fix: Corrects SubplotGrid indexing and sequential GeoAxes formatting

[cvanelteren]

Closes #356 and #358

This PR addresses two critical bugs introduced after v1.0: one causing incorrect indexing in SubplotGrid, and another causing sequential format calls on GeoAxes to override each other.

While this work is part of the larger effort in #349, this fix is being prioritized to resolve significant issues for users on the latest version.

Gridspec Indexing Bug

• The Issue: Slicing a SubplotGrid using 2D grid coordinates (e.g., axs[:, 0]) resulted in an incorrect selection of axes.
• The Fix: The SubplotGrid.__getitem__ method in ultraplot/gridspec.py has been reverted to the robust implementation from v1.0, which correctly handles array-like indexing.

Sequential Formatting Bug

• The Issue: When applying formatting to a GeoAxes object in sequence, the second call would override the first. For example, setting longitude labels with ax.format(lonlabels=True) and then latitude labels with ax.format(latlabels=True) would result in only the latitude labels being displayed.
• The Fix: The internal helper function _to_label_array in ultraplot/axes/geo.py was incorrectly defaulting unspecified label sides to False. It has been modified to use None instead, ensuring that only the explicitly passed labels are modified and preserving the state of other labels from previous format calls.

[codecov]

Codecov Report

❌ Patch coverage is 95.58824% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
ultraplot/gridspec.py	89.65%	1 Missing and 2 partials ⚠️

📢 Thoughts on this report? Let us know!

[cvanelteren]

Restored indexing, but found another bug. The subplot spec for panels are not correctly returned with this manner of slicing. Think this is a minor issue but should be addressed on the future.

The issue occurs for irregular layouts; otherwise it is not a problem.
fixed