Allow multiple copies of some tags

[mgalloy]

I'm not sure if all tags should be allowed to have multiple copies or just special tags. Restrictions and author are two that seem like they should allow multiple tags. The behavior here should probably be to append to the current value of the tag.

Other tags should give a warning if used twice, such as the boolean tags like abstract, obsolete, etc.

[mgalloy]

From jhood:

I noticed some discussion in the mailing list about multiple tags. My preference would be to have comments that come with the @uses tag accumulate into a list. (Right now the last one is used.) For example:

;  @uses utility_compile
;  @uses star_list_compile
;  @uses widget_compile
;  @uses algorithm_manager_compile
;  @uses resample_compile

would result in a comma delimited list rather than just resample_compile.

The advantage would be that used items / dependancies can then be more clearly seen and changed in the code too.