Match subject identifiers with and without the "sub-" prefix

[richford]

Problem

When using load_afq_data or AFQDataset, if the one of subjects.csv or nodes.csv has a "sub-" prefix for the subject identifiers and the other does not, then it won't find the matching phenotypic info for each subject.

Proposed solution

Add a drop_sub_prefix=False parameter to load_afq_data and AFQDataset. When False it should reproduce the current behavior. When True it should drop the "sub-" prefix from both the nodes and subject dataframes before merging.