make scipy dependency optional

[Liam-DeVoe]

Similar to our matplotlib dependency, scipy is quite heavy and only used for a single function (similarity with the correlation metric). Should be an opt-in dependency.

[Liam-DeVoe]

hm, I forgot that slider actually hard requires scipy. Would have to see if we can make it optional on their end first, which is either a fair bit of work or impossible, not sure which yet.