98 duplicate acsn bug resolved

[cthunes]

Duplicates acsn's have been removed from the assay_component_map table and the acsn column was given the UNIQUE constraint.

Added a check within tcplRegister for catching error and created output message if duplicates are attempted to be registered.

> tcplRegister(what = "acsn", flds = list(acid = 2938, acsn = "tox21-ror-cho-viability-p1_set1"))
acsn was not inserted into the database:
Duplicate entry 'tox21-ror-cho-viability-p1_set1' for key 'acsn' [1062]
Duplicate values are not permitted; this source may already be registered!
[1] FALSE

Closes #98.