Implement Debye Huckel Screening

[SamG-01]

Implements weak screening as per equation A1 of Chugunov 2009 (see below). Also adds an optional check against debye_huckel and computes the full screening factor if the check exceeds a supplied threshold, which can significantly improve runtimes (disabled by default).

[SamG-01]

[...] computes the full screening factor if the check exceeds a supplied threshold, which can significantly improve runtimes (disabled by default).

To quantify these runtime improvements, below is a benchmark of the subchandra simulation in Castro, with and without screening.enable_debye_huckel_skip, for several values of castro.react_rho_min. The stop time is 0.01 s, and screening.debye_huckel_skip_threshold is kept at its default of 1.01. Each set of runs was performed using four CPU cores of a Linux machine with 2 Intel(R) Xeon(R) Gold 5115 CPU @ 2.40GHz and a NVIDIA Titan V GPU.