Get conversion class

Created by: Durman

Problem statement

These lines are quite inefficient: https://github.com/nortikin/sverchok/blob/89b770b3b27805d85ba2f83f569a0e0897692ccc/core/socket_conversions.py#L320-L322 It takes 0.6% of whole execution time in a test tree but should takes none. Should be replaced with a dictionary I guess.