Added cortical action for "a = b * c"

This also covers a=b*~c and a=~b*c and a=~b*~c
This method looks pretty much identical to `add_conv_effect`. I think we should be refactoring some of this duplicated "SPA build logic" into a new location? I'm expecting more code sharing of this nature in the future, for instance when building complex hierarchical rules (i.e. nested convolution). So a good start might be to avoid this code duplication.