Clarify the purpose of Ei Market Context vs. EMIX Market Context

EI Market Context extends (by including) Emix Market Context. Editor may consider textual changes as he sees fit to reflect this. emix:MarketContextType acts as a UID for Market Contexts, assuming that the URIs are consistently applied. Re-excamine all payloads to ensure that the correct one is used. Add text to specification that an application miay choose to cache EiMarketContexts, but they not generally needed in operation payloads.