ablastr constants : add variable templates to allow for flexibility in using single/double precision

[lucafedeli88]

With this PR the constants in ablastr are first defined using variable templates and then specialized for amrex::Real, e.g., for mathematical constants:

    namespace math
    {
        namespace variable_templates
        {

            template<typename T>
            constexpr auto pi = static_cast<T>(3.14159265358979323846);

            template<typename T>
            constexpr auto tau = static_cast<T>(2.0 * pi<double>);

        } 

        constexpr auto pi = variable_templates::pi<amrex::Real>;

        constexpr auto tau = variable_templates::tau<amrex::Real>;

    } // namespace math

This is done with the intent of allowing the use of double precision constants even if the code is compiled in single precision (or vice versa if you really want it !), e.g. :

using namespace ablastr;
namespace math_vt = math::variable_templates;

const auto a = 2_rt * math::pi; // a is an amrex::Real

const auto b = 2 * math_vt::pi<double>; // b is a double !

I will need this feature to import the QED module from PICSAR into WarpX. I've started migrating the code in #5677 ,but the PR is becoming too big, so I decided to split it into smaller chunks.

The new feature is now used in the template function Algorithms::KineticEnergy . This function is conceived to be able to force double precision in some cases regardless of how WarpX has been compiled. Therefore, using c in double precision in these cases is coherent with its goal.

In case you don't like the long namespace name variable_template, an alternative would be to call it vt.

Note that I've changed the variables from static constexpr to constexpr because, to my understanding, static does not make any difference in this context.

[dpgrote]

Since this is for constants, perhaps call it constant_templates?