Add IS_COVARIANT and IS_CONTRAVARIANT properties to continuous dimensions

[EmilyBourne]

Add IS_COVARIANT, IS_CONTRAVARIANT and dual properties to continuous dimensions. E.g:

class X {
    /**
     * @brief A boolean indicating if the dimension is periodic.
     */
    static bool constexpr PERIODIC = false;

    static bool constexpr IS_COVARIANT = true;
    static bool constexpr IS_CONTRAVARIANT = true;

    using Dual = X;
};

class R;
class R_cov;

class R {
    /**
     * @brief A boolean indicating if the dimension is periodic.
     */
    static bool constexpr PERIODIC = false;

    static bool constexpr IS_COVARIANT = false;
    static bool constexpr IS_CONTRAVARIANT = true;

    using Dual = R_cov;
};

class R_cov {
    /**
     * @brief A boolean indicating if the dimension is periodic.
     */
    static bool constexpr PERIODIC = false;

    static bool constexpr IS_COVARIANT = true;
    static bool constexpr IS_CONTRAVARIANT = false;

    using Dual = R;
};

template<class Dim, typename = std::enable_if_t<Dim::IS_CONTRAVARIANT>>
using to_covariant_t = typename Dim::dual;

template<class Dim, typename = std::enable_if_t<Dim::IS_COVARIANT>>
using to_contravariant_t = typename Dim::dual;

[EmilyBourne]

@gdgirard Do you agree with these names? Specifically Dual to represent the matching covariant/contravariant type?

[gdgirard]

Dual seems fine for me. But maybe R_cov instead of R_co otherwise difficult to remember that _co is for covariant and not for contravariant

[EmilyBourne]

Dual seems fine for me. But maybe R_cov instead of R_co otherwise difficult to remember that _co is for covariant and not for contravariant

Sounds good. I have updated the issue