UMAP feature_weights refactoring

[alessiamarcolini]

_get_feat_weights method of an UmapExperiment (https://github.com/HK3-Lab-Team/umapviz/blob/master/src/umapviz/umap_exp.py#L460) is called to get the feature weights, and in case self.feature_weights = () they are calculated and then the result is set to the feature_weights attribute and it is returned, too. Is that right?

If so, why not having a feature_weights property who calculates the result? If performance is a concern, we can cache the result so subsequent calls to the property won't trigger the calculation.