New loss functions

[ahwillia]

Users should have the ability to specify different loss functions. A few easy ones:

• Logistic loss (spiking data where there is at most 1 spike per bin)
• Poisson loss (spiking data with multiple spikes per bin)
• Gamma loss (for calcium imaging data)

In the case of Logistic/Poisson loss functions, this would remove the need to smooth the data as a preprocessing step (see #2), but would necessitate adding regularization for smoothness on the temporal factors. This could also help with #3 - e.g. the reconstruction can be interpreted as a probability of spiking in the logistic case.