(re)add Excel reader

[simleo]

While refactoring the ICY reader (see #21) we removed a generic Excel to CSV converter, xls_to_csv. As noted in the PR comments, a generic Excel reader can be useful for other formats. We can either readd that function, perhaps in another module since it does not follow the same read-file-to-dataframes protocol as current readers, or develop a generic Excel reader based on it (writing to CSV would be accomplished by the data package writing code).