Change the implementation of the metadata structure

[lucananni93]

In this moment the Metadata structure in Pandas is implemented as a Dataframe of lists. But this makes the selection and filtering of samples based on metadata pretty difficult and verbose.

Change that needs to be done:

• Encapsulate the python list of metadata value in an object (called, for example MetaCell)
• Overload the operators in order to make the syntax coherent with GMQL
• Overload the toString function for good visualization in Pandas