Small performance improvements and memory leak fix

[leuty]

This PR reduces redundant I/O and computation across the plotting pipelines. The topography NetCDF data is now cached so it is read from disk only once rather than re-loaded for every subplot. Similarly, ATAB file parsing is cached across pipelines, and the files themselves are only read from disk a single time.

On the plotting side, a redundant tight_layout=True was dropped from the station scores subplot initialization since an explicit fig.tight_layout(...) with custom parameters is already called immediately after. s.

Unused imports were cleaned up as well.

I did not do any benchmarking, but staring at the logs seemed like the patches help a bit.

The PR is best reviewed commit-by-commit.