Implement logic_to_wt

[dglmoore]

The aim of #84 was to create generic network conversion functions, i.e. functions that, given a network of one type, would construct a network of a different type (if possible). We have the neet.boolean.conv.wt_to_logic function which serves this purpose for converting WTNetworks to LogicNetworks. We should have a function which goes the other direction as well logic_to_wt.

Proposed API

class ConversionError(Exception):
    pass

def logic_to_wt(net):
    pass