Metadata object

Aaaaand the last important object is in!! I was struggling with a couple of errors and a merge conflict, but it is finally here!!

The whole idea of the Sample/Prep templates objects is to behave as a dict of dicts, in which the first  level of keys are the sample ids and the second one are the metadata category headers.

Anyone available to review this??
It's not a direct dependency. pandas depends on openpyxl and on the last release of pandas (0.14.0) they require a version between 1.6 and 2.0. It looks like conda installs the latest version, which raises a warning in the builds