Bug/Feature request: current `IV_by_period()` has very limited return options

[martinctc]

Currently, the function IV_by_period() can only return a "table" option. There are many other viable return options, including the original IV object, a plot, or a list of the WOE tables. We can reference create_IV() to return a consistent set of outputs.

Have marked this function as experimental life stage in #67 for this reason.