{pyactr} Support declarative memory's finst_time

[asmaloney]

From my comments in the code, these are the current defaults:

// "finst_time": how long the finst lasts in memory
// 	ccm (finst_time): 3.0
// 	pyactr: (unsupported? Always ∞ I guess?)
// 	vanilla (:declarative-finst-span): 3.0

This pyactr issue suggests how to add it over there.

If it is added to pyactr, then update gactar to handle it properly.