`AsyCost` doesn't work for generalized index spaces

[Krzmbrzl]

As can be seen in

SeQuant/SeQuant/core/asy_cost.hpp

Lines 13 to 28 in 44ce3e5

	///
	/// Represents a symbolic asymptotic cost in terms of active_occupied
	/// and the rest orbitals.
	/// eg.
	/// - AsyCost{2,4} implies scaling of $O^2V^4$. In other words, the cost
	/// scales by the second power in the number of active_occupied orbitals
	/// and the fourth power in the number of the rest orbitals.
	/// - AsyCost{sequant::rational{1,2}, 2, 4} implies the same scaling as
	/// above except the numeric value obtained by substituting $O$ and $V$
	/// numbers is then halved.
	///
	class AsyCost {
	private:
	class AsyCostEntry {
	size_t occ_; // power of active_occupied
	size_t virt_; // power of the rest orbitals

the AsyCost class is hardcoded to work for exactly two index spaces that can be categorized into occupied and virtual.

This model breaks down in multireference theories where expressions will involve at least a third index space (active). And for more general cases (e.g. mixing of fermionic and bosonic vector spaces) this will obviously fail as well.

To be more general, the class should eventually be refactored to use a map<IndexSpace, std::size_t> instead of explicit member variables for every space.