Feature
* feat: add sparse PCA using variable projection (170)
Publication of this method is available at http://arxiv.org/abs/1804.00341. Associated repository with Python code can be found here: https://github.com/erichson/ristretto/tree/master/ristretto ([`e2ccc76`](https://github.com/xarray-contrib/xeofs/commit/e2ccc7621c33f55a1ca4c2afe588123ea12deffa))