aaxi: Axisymmetric Numerical Relavitity

aaxi implements an formulation derived from the axisymmetric evolution scheme of Rinne 2006 to second-order in space, first-order in time using aeon-tk. This scheme is purely hyperbolic during evolution, and solves for initial data using a hyperbolic relaxation solver (modelled after NRPyElliptic's solver). This is significantly more complex and numerically expensive than spherical symmetry.