Expand *_layer to allow dynamic layer selection

[finnlindgren]

To handle space-time covariates where each timepoint is stored in a layer, should let main_layer take an expression of inputs that computes a vector of layer indices. The internal spatial grid/raster evaluator would then need to use this when extracting information.

[finnlindgren]

Alternatively, replace the current special handling with an evaluator function that could be called as comp(evaluator(covariate_object, cbind(x,y), timepoint), ...). With this approach, future enhancements wouldn't require changing the available component() arguments, but could instead just provide new/different evaluators.

[finnlindgren]

This has been implemented via the eval_spatial generic.