add utility function allowing to sparql-escape numbers as xsd:decimal

[MikiDi]

Since mu-cl-resource's default datatype for numbers is xsd:decimal, this could prove to be a useful addition for setups that write numbers through mu-cl-resources as well as through custom services based upon the JS template. This way, one uniform datatype can be used for all occurrences of a certain value.

[erikap]

This is a good addition @MikiDi :pray: We're looking for a strategy to make this happen in all templates. Progress is tracked in https://github.com/mu-semtech/project/issues/1