2.0.0rc6

This is another update in the ongoing process of moving to specparam 2.0.

This update continues on from rc5, with the main update being the addition of a generalized procedure for doing post-fitting parameter conversions, including options to specify different calculations of peak heights. The default behavior has not changed, but the user can now optionally specify which parameter conversions to apply (from built in options) and/or provide custom parameter conversions. See more details in PR #383, as well as in the new docsite materials.