Error during collection creation with a different id_name

[flahn]

When you create a collection for a process graph with a customized id name, then it will come to an error:

 Error in process(., "filter_daterange", prior.name = "imagery", from = "2017-04-01",  : 
  Chain corrupted. prior elemente is neither a process or a collection 

It means that a specific key word for process or collection cannot be found. I will change this implementation to use an attribute for this like the follwing to allow independent type checks.

attr(x, "type") <- "process"
attr(y, "type") <- "collection"
attr(z, "type") <- "udf"

[flahn]

fix this for Open-EO/openeo-r-backend#3