implement opt_einsum_dask_{single,threads} backends in ETermBase.make_function()
implement opt_einsum_dask_{single,threads} backends in ETermBase.make_function()

- support backend kwargs in .set_backend()

BubuLK/sfepy,BubuLK/sfepy,rc/sfepy,rc/sfepy,sfepy/sfepy,vlukes/sfepy,vlukes/sfepy,BubuLK/sfepy,sfepy/sfepy,rc/sfepy,vlukes/sfepy,sfepy/sfepy