Add statistic for how many values have the same meaning but are different.

[ypid]

Using the isEqualTo function. https://github.com/ypid/opening_hours.js/issues/116

It is expected that this calculation has high demands in term of CPU time. The calculation will be done only once a year. Before doing this it might be a good idea to find a way to make use of multiple CPU cores using NodeJS.