Support Functional layer

[vincentsarago]

https://github.com/developmentseed/tifeatures/blob/master/tifeatures/layer.py#L197-L215

[vincentsarago]

in #8 we will unify the table_catalog, but functional_layer can't be the same between timvt and tifeatures (because the function returns different output type) We should then make sure the function_catalog to have specific name: tifeatures_funtions_catagog