Add optional start situation ID parameter to the context API

[noracato]

Right now we cache results for the analysis of the ESDL start situation based on its ID. When an API request is made, the ID is read from the first lines of the start-ESDL and the results are grabbed form the cache. To further optimise for gridmaster, we'd like to add the posibility to just supply this ID instead of a whole ESDL for the start situation. If the ID is still in the cache, we can use the results, otherwise, the API should ask for more information (via a specific response?)