change slice sampler parameters to match HDDM

[frankmj]

Reports that slice sampler is quite slow - makes sense that it is slower than the NUTS sampler but it is also a lot slower than the one in HDDM. I believe this is because in HDDM we tuned the slice sampler parameters so that they were much more effective for DDM (and it seemed to work fine for other SSMs/ LANs), so one very simple thing to do is to just reuse the slice params from HDDM to be default for HSSM slice.

[AlexanderFengler]

First tests do not show any significant impact of setting slice widths per parameter.
More testing required.