Add occurrence count in the json output for search engine

[panglesd]

Since #972 there is a compile-index command which outputs a json index containing all searchable entries. The intended purpose of this json index is to pass it to a search engine.

Since #976 there is also a count-occurrences command which creates a table containing the number of occurrences for each identifier.

This PR makes the compile-index be able to use the information from count-occurrence. It adds a --occurrences argument to get the path to the table.

The first commit isolate the occurrence code in its own library. The second commit implements the --occurrences argument of ``compile-index`.

[panglesd]

Note that there will be a conflict with #1067 which we will need to resolve (in whichever PR gets merged last).

[EmileTrotignon]

What's the status of this ?

[panglesd]

It was ready for review. Now there are many conflicting files.

[panglesd]

It is now rebased and ready for review.