Expose AMS iterations in config and update some AMS options

[simlapointe]

Tests have shown that increasing the number of internal AMS iterations and the number of multigrid cycles, as well as changing AMS's coarse relaxation type leads to a significant reduction in the number of GMRES iterations and a noticeable reduction in overall runtime. Specifically, this PR:

• Exposes the number of AMS iterations in the config file (config["Solver"]["Linear"]["AMSMaxIts"]).
• Changes the default number of AMS iterations from 1 to the solution order (config["Solver"]["Order"]).
• Changes the default number of multigrid cycles (config["Solver"]["Linear"]["MGCycleIts"]) from 1 to 2 when AMS is used as the coarse solver.
• Changes the AMS coarse relaxation type to direct Gaussian elimination.