Support mixed formulas and columns in By Code

[fredericDelaporte]

Many mapping elements allow to define their columns or formulas by nested elements, but most of them were not supporting having both nested columns and formulas.

Follow up to #1808, which has added this support for hbm mappings.

Fixes #1278 by the way.

[hazzik]

Can we come up with a better name other that "ColumnsAndFormulas"? Or some other method how to declare these?

[fredericDelaporte]

The way mapping by code is designed, I have no better idea.
Current existing methods are void Columns for mapping many columns, void Column for mapping a single column and void Formula for mapping a single formula. None of them allows for chaining.

So for mapping many columns and formulas, a void ColumnsAndFormulas method seems quite consistent with the current API shape.

Introducing something like MapMultiple().Column(...).Formula(...).Column(...) would be very inconsistent with the current API shape.

[fredericDelaporte]

Since the mapping by code naming seems to block this issue, I think the hbm mapping fix should be merged separately. I will add a separated PR.

[fredericDelaporte]

I still do not have a better name. In fact, I do not think there is a better name fitting the current API shape, which already does not follow usual guidelines for method names. (For example, they are not prefixed by an imperative verb.)