Inherit raise_on_mismatches from the superclass

[afurm]

Fixes #174.

raise_on_mismatches is stored in a class instance variable, so setting it on a shared base experiment class didn't apply to its subclasses:

class BaseExperiment
  include Scientist::Experiment
end

class WidgetExperiment < BaseExperiment; end

BaseExperiment.raise_on_mismatches = true
WidgetExperiment.raise_on_mismatches? # => nil, mismatches are silently published

This makes raise_on_mismatches? fall back to the superclass when the class hasn't set its own value. A subclass can still opt out with raise_on_mismatches = false.

I added tests for both cases and a line to the README. The full suite passes locally except includes the backtrace when an observation raises, which also fails on main under Ruby 3.4+ because of the new backtrace quote style.