This is an author-created, un-copyedited version of an article published
in Nonlinearity. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or
any version derived from it. The Version of Record is available online at https://doi.org/10.1088/1361-6544/aa88ff.
The work of K.S. was supported by the EPSRC as part of PhD study at Loughborough.