Support PCA on a subset of features but the full rotation matrix is still reported.

[LTLA]

Closes #4.

[codecov]

Codecov Report

❌ Patch coverage is 98.59155% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
tests/src/subset_pca.cpp	97.76%	4 Missing ⚠️
include/scran_pca/simple_pca.hpp	94.11%	0 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
include/scran_pca/blocked_pca.hpp	93.77% <100.00%> (-3.46%)	⬇️
include/scran_pca/subset_pca.hpp	100.00% <100.00%> (ø)
tests/src/compare_pcs.h	81.39% <100.00%> (-2.82%)	⬇️
include/scran_pca/simple_pca.hpp	91.39% <94.11%> (+0.28%)	⬆️
tests/src/subset_pca.cpp	97.76% <97.76%> (ø)

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