BQS — issue: adding the source of subject headings

The current spec says that the subject headings come from standard
lists (e.g. SEARS, LCSH). But the spec does not have an attribute
specifying this source. After consulting with the librarians and similar
people, we found that this information is so important that it should
qualify for having its own and explicit attribute.
This is what was shown in the last resolution as
public string subject_heading_collection;
The contents of this attribute could be, for example, "SEARS", "MeSH",
"LCSH".

This, of course, opens the question why we are not suggesting the same
for classification codes. We don't because we feel that classification
codes are unique, they are more or less like identifers - and therefore
they can be even expressed as identifiers using the same notation as with
databases (source/id).
But the subject headings are not necessarily unique: for example
something like "MeSH/blahblah" can appear in several places in the MeSH
trees) - and therefore using for that the pattern used for identifiers
would not be appropriate. Therefore, the example just above would be coded
as "blahblah" as one of the 'subject_heading' strings, and the word "MeSH"
woud be in 'subject_heading_collection'.

Resolution:
To add subject_heading_collection (as shown already).
To clarify documentation about differences between subject headings and
classification codes, with a suggestion that for classification code may
be used pattern for identifiers.

see below