Lock-in to Jena in core

core/helpers/RequestHelper uses [jena UpdateRequest](https://github.com/janeirodigital/shapetrees-java/blob/169cb800e378365ebbd969d16048bc83b88cb98d/shapetrees-java-core/src/main/java/com/janeirodigital/shapetrees/core/helpers/RequestHelper.java#L201-L202), which parses and executes the resquest. Do we want an abstraction for platforms not using Jena?