Extension to Slater-type orbitals

Currently GauXC is limited to Gaussian-type basis functions $$ R_i(r) \propto r^{l_i} \sum_i c_i \exp(-\alpha_i r^2) $$ but it shouldn't be too difficult to extend support to Slater-type orbitals $$ R_i(r) \propto r^{n_i-1} \exp(-\zeta_i r) $$ where $n_i$ may be non-integer.