[REF] Refactor load_data for gifti images with more than one data array

[VinceBaz]

This PR refactors the load_data function to allow it's use for gifti images with more than one data array (e.g. for timeseries/functional data). There were a few issues with the function, which are related to how the agg_data method for GiftiImage objects (from nibabel) handles the intents of data arrays: when the intents of a data array is set to NIFTI_INTENT_TIMESERIES, the agg_data method returns a 2D ndarray, but when the intents are something else (e.g. NIFTI_INTENT_SHAPE), then the data arrays are returned as a tuple of multiple 1D array. The load_data was not working as expected in either of these cases (i.e. it couldn't load nor stack data images into numpy arrays).

This PR should fix Issue #139