Add whosmat() to list variables without loading data

Brurequested to merge
bruAristimunha/pymatreader:add-whosmat-function into master

Add a unified whosmat() function that lists variables (name, shape, class) in MATLAB files across all versions. Delegates to scipy.io.whosmat for v4-v7.2 and uses a new _whosmat_hdf5() helper for v7.3 (HDF5) files.

Closes #23 (closed)

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