pystencils: re-allow use of arbitrary field names in sweeps

Previously, anything that wasn’t velocity, src, src_tmp, pdfs, or pdfs_tmp resulted in invalid code, e.g.

auto u = block->getData< field::GhostLayerField<double, 3> >(uID);

auto & eta = this->eta_;
auto & u = this->u_;

instead of

auto u = block->getData< field::GhostLayerField<double, 3> >(uID);

auto & eta = this->eta_;

This was a regression introduced in !259 (merged).