Replaced all instances of "key" with "training" (Issue #21)

Replaced all instances of "key" with "training" except where "key" was a
part of "keys" as  a reference to the tab name, which I think I'm gonna
just call "Training" instead of "Tranings" or "Training Records"... I
haven't decided.
Updated tab name where applicable to "Training" (Issue #21)

Updated tab name references to "Training".
Replaced all instances of "$k" variable name with "$t" for consistency (Issue #21)

There are a few places where the variable $k was used.  Replaced it with
$t to be consistent.