Allow intercept-free models

[nicholasjclark]

The formula interface should allow for simple SS models where there is no intercept in either the observation or the latent state models. Something like:

   formula = y ~ -1,
   trend_formula = ~ -1, 
   trend_model = 'AR1',
   ...

Because we need an underlying (and valid) mgcv model for generating predictions, we'll need to include the intercept in the model and either fix it at zero (as a transformed parameter) or regularize it to zero

[nicholasjclark]

This is all done now in the latest versions