add method getWidthHistory for slice sampler (#1057)
add method getWidthHistory for slice sampler (#1057)

* add method getWidthHistory for slice sampler

- add option to slice sampler to save sampler history (width) following the MCMCsaveHistory argument and the way it was done for RW sampler
- add method getWidthHistory to slice sampler to retrieve the width history

* Update MCMC_samplers.R

Edited reset method in slice sampler to reset widthHistory as well
nimble-dev/nimble,nimble-dev/nimble,nimble-dev/nimble,nimble-dev/nimble,nimble-dev/nimble