Implement listNeighbors

[dehann]

listNeighbors(clicon..., "x4")

get back the list of factors connected to that variable

["x3x4f_sdr"; "x4x5f_3r3"; "x4l7f_34r"]

And visa versa for the variables connected to a factor, but that case should just alias to equivalent of:

getVariableOrder(dfg, fctlbl)

which gives back

["x3";"x4"]

Thereafter, alias the specific dispatches for list and ls, also lsf.

PS, I'm writing hot and loose wrt String vs Symbol -- should resolve the default there better.

[dehann]

xref:

• Implement listNeighbors #65