Review structure of metadata

We should review the structure of the metadata, and in the master table:

• new substantially version of the master table (all "ready" indices should hopefully be OK) (cf. #70 (closed))
• separate index definition and index function information, and "global metadata" in the metadata.yml (cf. #18 (closed))?
• move index function definition metadata part into the index function, thus help templating (cf. #62 (closed))?
• updated master table may help automating the index definitions (e.g. grandfathering #7 (closed) and !41 (merged))
• Representation of simple stats. and thresholded stats. in the master table (column "index_function"), and as index_functions (cf. #62 (closed), #63 (closed))