Record the execution context declared by the computation library #3

Problem

The recorder builds the execution context of an activity from the description of the call given by the computation library (describe_call), but keeps only two fields of it: the global options read (options) and the metadata of inputs read (metadata_read, recorded as digests). Any other rule the computation depends on and that the library declares cannot be recorded, so that a replay cannot check it.

This is needed by Sigima to record the convention used to select image pixels in regions of interest (pixel centers since Sigima 1.4): activities recorded with an earlier convention must be refused explicitly (unsupported_context) instead of being reported as different.

Proposal

When the description holds a context mapping, the recorder adds its entries, as plain JSON data, to the execution context of the activity. A replay already passes the whole recorded context back to the library (prepare_execution(..., context=...)), which checks it. Without such a mapping (current Sigima versions), nothing changes. No change of the ledger schema (context is a free object).