Avoid writing duplicate acsns when they are already in the database

[cthunes]

If assay component name is different from what it is expected, it is possible to duplicate it under a different name. In the tcplRegister function if what = "acsn" we need code to make sure there is no other acsn with identical name.

Column should be unique in the ddl and there should be an informative error message when you try to register multiple assays with identical acsn