list_instruments depends on hard-coded values, therefore, not user friendly

[Julian Lam]

As described, the list_instruments function takes one variable, $instrument,
which is explicitly defined inventory.php

If new instrument classes are acquired by the cage, there is no way they can be
added to the cage system, unless the direct code is altered by hand.

Perhaps a table can be made to record the instrument classes, including
additions, deletions, etc...

It would also let the end-user filter the inventory based on what they're
looking for.