This class represents an operator used in predicates for
Denial Constrains (DC) representation. Predicates used there
are less, greater, eq, neq, geq and leq.

This C++ implementation is 100% bad, there is whole bunch of
objects being created, but conceptually all of them are the same.
Object "Operator '+'" and another object "Operator '+'" represents
the same thing.

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This is just a copy of a java code from https://github.com/RangerShaw/FastADC.

I will refactor and think about better implementation later.
I'll be just copying java code to get working algorithm ASAP,
and after that I'll start thinking about good implementation.

This commit adds test_dc_structures.cpp file, which will be used
to test different data structures which are required for
DC representation (there are a lot).

This class represents a column operand within a predicate for FastADC.

FastADC processes Denial Constraints (DCs) that involve comparisons between
pairs of rows within a dataset. A typical DC example, derived from a Functional
Dependency (FD) such as A -> B, is expressed as: ∀𝑡, 𝑠 ∈ 𝑟, ¬(𝑡.𝐴 = 𝑠.𝐴 ∧ 𝑡.𝐵 ≠ 𝑠.𝐵).
This denotes that for any pair of rows in the relation, it should not be the case
that while the values in column "A" are equal, the values in column "B" are unequal.

A predicate in this context (e.g., 𝑡.𝐴 = 𝑠.𝐴) comprises three elements to be fully
represented: the column operand from the first tuple ("t.A"), the comparison operator
("="), and the column operand from the second tuple ("s.A"). The `ColumnOperand` class
encapsulates the column operand part of a predicate, such as "t.A" or "s.A".

First step in FastADC algorithm is to build so-called "Predicate Space".
This is a long process during which many places in the code wants to get
a Predicate. But each predicate is stored in a global storage -- map.
In Java code this class (and other similar "provider" classes) are
singletons.

BaseProvider class is the class, from which a *Provider class should be
derived. It ensures that only a PredicateBuilder class can initialize
and free these singletons.

I'm sure there exists a better approach, where we will store Provider
classes in some fields to bind their lifetime more explicitly, but
this is how it's done in Java, and I don't have much time to devise
perfect architecture.

This class acts as a centralized storage to manage
and provide access to Predicate objects.

A Predicate is defined as "t1.A_i op t2.A_j", where t1 and t2 represent
different rows, and A_i and A_j are columns (which may be the same or different)

The FastADC algorithm first will build a so-called "Predicate Space",
which is a set of all predicates that are allowed on R (set of rows,
basically a table). In order to create and store predicates, this commit
implements a singleton class with a hashmap storage.

FastADC processes Denial Constraints (DCs) that involve comparisons between
pairs of rows within a dataset. A typical DC example, derived from a Functional
Dependency such as A -> B, is expressed as:
`forall t, s in r, not (t.A = s.A and t.B != s.B)`
This denotes that for any pair of rows in the relation, it should not be the case
that while the values in column "A" are equal, the values in column "B" are unequal.

A predicate in this context (e.g., t.A == s.A) comprises three elements to be fully
represented: the column operand from the first tuple ("t.A"), the comparison operator
("="), and the column operand from the second tuple ("s.A").

This simple test creates two predicates on a 2x2 table and evaluates them.
We're checking for mo::GetPredicate function ability to correctly
create a predicate

In the original FastADC pull request this class manages creation of
predicates, so it initializes PredicateProvider. But in this pr this
class is not required for DC verification. Hence adding a temorary
class just to make the tests work