Introduce `GetDecimal()` in preference to `GetDouble()`

[JeremyCaney]

When retrieving attributes, we have a number of strongly typed extension methods for retrieving the attributes as specific types. Currently, we have GetInteger() and GetDouble(). But, in practice, most of the places our clients are using GetDouble() would be better suited by GetDecimal(). To facilitate this, introduce a new GetDecimal() extension method. We can keep the GetDouble() in case it meets client needs, though we will generally expect GetDecimal() to be preferred.