least squares loss

[ericstrobl]

Request for the least squares loss to handle non-Gaussian continuous data with linear relations as described in Equation 1.3 of this preprint: https://arxiv.org/abs/1511.08963

[itsrainingdata]

This has been implemented for advanced users in the ccdrAlgorithm as of itsrainingdata/ccdrAlgorithm@6bbafda. As of now, this is only available through ccdr.run. This will eventually be ported to sparsebn inside of estimate.dag.