#20 Better ways to get and set triples on stores

1. Add new function `LSUP_store_lookup()` which does the same as the current `LSUP_graph_lookup()` within a store context (with an added, optional `c` argument). 2. Change `LSUP_graph_new()` implementation so that if `NULL` is passed for the `c` arg, and if the store supports contexts, `c` is set to the store's default context (anonymous graph). In summary: - `LSUP_graph_get()` is used to get a complete named graph from a a store. - `LSUP_store_lookup()` is used to generate a graph from triples in any context, using arbitrary lookup criteria. - `LSUP_graph_lookup()` is used to generate a sub-graph from a graph.