Create module chado_linker_fields

[dsenalik]

This issue is to document the creation of this module.

This module has been created in response to this issue in the Tripal repository tripal/tripal#1318 and the related pull request tripal/tripal#1319 - and there was an earlier discussion about analysis-project links in tripal/tripal#804

So some background, a fair number of content types have chado linker tables that define relationships between two different content types. For example, in the core Tripal module there is a field named chado_linker__contact that can be used, for example, to add contact information on a feature, such as a gene. The correspondence information is stored in the chado table feature_contact. Well the question was raised as to why aren't there linker fields for all of the major content types. And the answer was basically that no one wanted them enough to take the (significant) amount of time to create them. Well, some people did, but they were implemented in other modules, for example the sep__biological_sample field (Biomaterial Browser) in the tripal_biomaterial module.

Alas, @dsenalik wanted them enough, and began the arduous task.

Complications arose, some modules defined their own linker tables, sure that's great, but they conflicted with core tripal tables. For example, mainlab tripal defines stock_project which is in conflict with the core tripal project_stock table. Nasty special-case code tries to favor the core Tripal tables in these cases.

Another issue that caused chaos was that some of these linker fields made pages unviewable, because there were too many links. For example, if all the genes (features) in a gene prediction analysis are linked to the analysis, you have tens of thousands of links. Any Tripal site is going to roll over and play dead in such a situation. To handle cases of large numbers of links, the fields implement a pager, but when there are just too many records to display without the site timing out, the fields will switch over to a Summary View when the total number of records exceeds a configurable limit, defaulting to 200 at the moment.

And what about a Tripal 4 version? The plan is for these fields to be in core Tripal, or the new genome module, or other module, as appropriate. For now there is tripal/tripal#1381