read_dist() should check whether the file provided is a lower triangular distance matrix, a square matrix, or not a valid format.

Details on phylip-formatted distance matrices: https://mothur.org/wiki/phylip-formatted_distance_matrix/#:~:text=The%20basic%20format%20of%20a,sequence%20to%20the%20other%20sequences.

Note: phyloseq::import_mothur_dist() exists, but we should have our own version because:

• That function returns a matrix, but we like dataframes.
• We don't want to have many external dependencies.