spmodel 0.2.0

Minor updates

splm() and spautor() allow multiple models to be fit when the spcov_type argument is a vector of length greater than one or the spcov_initial argument is a list (with length greater than one) of spcov_initial objects.
- The resulting object is a list with class spmod_list. Each element of the list holds a different model fit.
- glances() is used on an spmod_list object to glance at each model fit.
- predict() is used on an spmod_list object to predict at the locations in newdata for each model fit.
Added the splmRF() and spautorRF() functions to fit random forest spatial residual models.
- The resulting object has class spmodRF (one spatial covariance) or spmodRF_list (multiple spatial covariances)
- These objects are built for use with predict() to perform prediction.
Added the covmatrix() function to extract covariance matrices from an spmod object fit using splm() or spautor().
Minor vignette updates.
Minor documentation updates.

Fixed a bug that prevents display of spatial covariance type in summary of spmod objects.
Fixed a bug that prevented prediction of factor variables when all levels of all factor variables did not appear in newdata.