[c++] Column abstraction: SOMADimension, part 1

[XanthosXanthopoulos]

SOMAColumn provides an abstraction over TileDB attributes and dimensions and exposes a common interface for all columns regardless of type. Subclasses of SOMAColumn can implement complex indexing mechanism through additional dimensions and encapsulate all that logic in one place and make it modular.

Subsequent PRs will add implementation for dimension and attributes.

Throughout this PR there is extensive use of std::any to enable polymorphism with the different SOMAColumn types while maintaining a templated interface at the abstract SOMAColumn.

Notes for Reviewer:
This PR introduces the abstract SOMAColumn class and the SOMADimension concrete class with basic unit tests.

[johnkerl]

I'm only partway through this PR. I'm still struggling with this (now) 1,875-line PR -- it's doing a lot of things. I'll need to pause and work on some other things for a while, and come back to this. (The other 3 PRs you split out that were smaller were nicely self-contained and self-descriptive and I was able to understand and review them earlier today.)

[johnkerl]

There was a comment here about why both \x7f and \xff -- it needs to be retained. This is very confusing without that comment. (Which is why I put the comment in.)

TileDB-SOMA/libtiledbsoma/src/soma/soma_array.h

Lines 901 to 920 in a8349a5

	// Here is an intersection of a few oddities:
	//
	// * Core domain for string dims must be a nullptr pair; it cannot be
	// anything else.
	// * TileDB-Py shows this by using an empty-string pair, which we
	// imitate.
	// * Core current domain for string dims must _not_ be a nullptr pair.
	// * In TileDB-SOMA, unless the user specifies otherwise, we use "" for
	// min and "\x7f" for max. (We could use "\x7f" but that causes
	// display problems in Python.)
	//
	// To work with all these factors, if the current domain is the default
	// "" to "\7f", return an empty-string pair just as we do for domain.
	// (There was some pre-1.15 software using "\xff" and it's super-cheap
	// to check for that as well.)
	if (arr[0] == "" && (arr[1] == "\x7f" || arr[1] == "\xff")) {
	return std::pair<std::string, std::string>("", "");
	} else {
	return std::pair<std::string, std::string>(arr[0], arr[1]);
	}

[jp-dark]

This looks good for the most part, but it's so big it's hard to follow everything. Let's do a code walk through together next time we are both working.

I also added a couple small comments, and John's comment about the missing code comments still needs to be addressed.

[jp-dark]

Why virtual inheritance? As far as I can tell, nothing inherits from SOMADimension.

[jp-dark]

Looks like you remove these in a future PR. Might as well just not add them in to begin with.