Error budget for rhealpixdggs-py

[ndemaio]

Raised by @rggibb during another discussion:

One thing I have been aware of for a long time that we havent implemented is that OGC AS Topic 21 v2/ISO 19170.1 says that every DGGS shall have an error budget as described in Section 9.1.5.3. Cells of equal area. The error budget controls both the precision that the equal area test is computed to and also the maximum practical cell depth that is supported by the DGGS. The error budget is a mixture of both the precision to which the ellipsoid is defined and the limits of precision in the implementation code. Without having looked at it it in great detail, it would seem to me that this work highlights the precision of the implementation and is an opportunity for us to be explicit about the error budget we are using and as a consequence the practical depth ie number of cell tesselation iterations, or smallest area cell that the code and therefore the DGGS supports.

He provided some additional content, pointing out that the comment

is a consequence of this code base preceeding the writting of OGC AS Topic 21

So adding a error budget to the code base will implement a missing feature and bring the package closer to the DGGS standard.

[alpha-beta-soup]

See https://docs.ogc.org/as/20-040r3/20-040r3.html#toc29 §9.1.5.3