Translation of BCs to primatives

[xtalax]

At the moment whole equations are recognized as belonging to a particular boundary, or being a type of interface between regions.
We want a method for seeing whether they are dirichlet, robin, neumann, periodic or interface as many subpackages will rely on these primatives.

This should be robust to different forms, i.e. u(0, t) - 1 ~ 0 and 0 ~ 1-u(0, t) should be recognized as dirichlet BCs, not robin, and equivalent to u(0, t) ~ 1

Should have the answer stored in a few types of struct such as:

struct DirichletBC <: AbstractAtomicBC
    """
    The dependent variable of the BC (e.g. u(0, t))
    """
    u
    """
    The independent variable of the BC (e.g. x), where this is the one held constant
    """
    x
    """
    Where this bc is defined, unused for now but will be important for exotically shaped domains
    """
    support
    """
    The RHS
    """
    expr
end