A set of operations on keys that you need to implement in order to use the
 `Ranges<'TKey>` type. The `'TKey` type is typically the key of a BigDeedle
 series. It can represent different things, such as:

  - `int64` - if you have ordinally indexed series
  - `Date` (of some sort) - if you have daily time series
  - `DateTimeOffset` - if you have time series with DTO keys

 The operations need to implement the *right* thing based on the logic of the
 keys. So for example if you have one data point every hour, `IncrementBy` should
 add the appropriate number of hours. Or if you have keys as business days, the
 `IncrementBy` operation should add a number of business days (that is, the
 operations may be simple numerical addition, but may contain more logic).

 Provides F# functions for working with the `Ranges<'T>` type. Note that
 most of the functions are also exposed as members. The terminology in the
 functions below is:

  - **offset** refers to an absolute `int64` offset of a key in the range
  - **key** refers to a key value of type `'T`

 Say, you have daily range `[ (2015-01-01, 2015-01-10); (2015-02-01, 2015-02-10) ]`.
 Then, the keys are the dates and the offsets are 0 .. 9 for the first part and
 10 .. 19 for the second part.