Implement LNP / GLM model

[billbrod]

In order to allow users to move from nemos to plenoptic, need to implement a pytorch version of the LNP model, and make it easy to get the parameters from a model that has been fit with nemos. Should test against the nemos version to ensure output is the same. Make sure have same choices for non-linearity and observation model, but regularizer and optimization algorithm don't matter.