Support for statistical aggregate functions

[austindrenski]

PostgreSQL supports a number of statistical aggregate functions that could be mapped to provider-specific extensions as in #323 and #407.

[roji]

That sounds great - any translations to SQL we can add improve the provider.

However, I'm not sure Math.NET is the right way to go here. The functions are both are pure calculations, providing as a result simple numbers. It would be easy to simply add them to EF.Functions, without forcing a plugin (and also Math.NET itself) on users, and be done with it.

The real value of plugins in the case of NodaTime and NTS is that they provide access to actual types that can be written and read from the database (and then used further client-side in the application). In this case. No such thing exists here - just a bunch of math functions to be translated. Another argument against Math.NET here is that it's likely to contain many more functions beyond what PostgreSQL can provide (I'm assuming), so the provider will only be able to translate a subset of them, which would be a bit confusing and not very good for discoverability.

We could also look into writing a plugin for Math.NET Numerics, which would provide client-side implementations when translation is not available.

If users want to use any functionality client-side, they can do so without any sort of support from the provider (that's the point of client-side evaluation :)).