WIP: Add Neurovault meta-data (mainly 2nd Level) as attributes 

This is a proposal to add attributes to encode some of Neurovault meta-data including:
- `groupComparison`: Is this a group comparison?
- `repeatedMeasures`: Is this a repeated measures design?
- `designType`: Type of model (one-sample, two-sample, regression)
- `smoothingFWHM`: FWHM of Gaussian smoothing kernel
- `groupInferenceMethod`: Inference method (random-effects, fixed-effects, mixed-effects)
- `estimationMethod`: Estimation method (Ordinary Least Squares, Generalised Least Squares, …)

This PR also includes data modelling as proposed in PR #32. (PR #32 should be reviewed first)