MINOR: stktable: add table_{inc,clr}_gpc* converters

MINOR: stktable: add table_{inc,clr}_gpc* converters

As discussed in GH #2423, there are some cases where src_{inc,clr}_gpc*
is not sufficient because we need to perform the lookup on a specific
key. Indeed, just like we did in e642916 ("MEDIUM: stktable: leverage
smp_fetch_* helpers from sample conv"), we can easily implement new
table converters based on existing fetches. This is what we do in
this patch.

Also the doc was updated so that src_{inc,clr}_gpc* fetches now point to
their generic equivalent table_{inc,clr}_gpc*. Indeed, src_{inc,clr}_gpc*
are simply aliases.

This should fix GH #2423.