Draft: Construct the covariate attribute for Dataset objects

This MR is a preparatory MR for the addition of covariates handling in Leaspy models. Two main changes are provided in the MR.

What does the code in the MR do ?

• 8c76ada5: renames all occurrences of cofactor by covariate, both in the code identifiers, comments, documentation and data. This solves #73
• 7aa74910 (and 113b98d7 for the corresponding tests): allows Dataset objects to instantiate and correctly load a covariate attribute, from the covariates stored in the Data object