User hook for command DUMP

The command DUMP displays the content of one or all sections of the R buffer. When CLASS finds a user section to dump, it can display the information it knows about each subsection. What it can not do is guessing what the data block contains.

If the section owner wants to tell CLASS how to dump its subsection, it has to:

1. tell CLASS which subsection it owns with class_user_owner. This is optional if it has been done elsewhere.
2. declare to CLASS with class_user_dump a subroutine which is able to display what is in the data block.

If CLASS tries now to DUMP the user section, it will loop over all the subsections it contains. If one matches the owner+title couple, it will put the data block in a specific place, and then call user's dump subroutine. This subroutine will then just have to translate back the data block to its own data type.

This last point must be fulfilled by a subroutine exactly symmetric to the transfer routine used at write time, i.e. elements should be reread in the same order they have been written.