Command line override args

[alwinm]

This minor patch allows command line overrides, which is a quality-of-life convenience when testing and can reduce the # of input files needed to run a variety of tests. Use examples:

Changing the output directory
./bin/cholla tests/3D/constant.txt outdir=dir2/

Changing the resolution
./bin/cholla tests/3D/constant.txt nx=100 ny=100 nz=100

Changing tout
./bin/cholla tests/3D/constant.txt tout=1000.0 outstep=1000.0

Changing parameters
./bin/cholla tests/3D/constant.txt rho=1e2

Everything combined
./bin/cholla tests/3D/constant.txt outdir=dir2/ nx=100 ny=100 nz=100 tout=1000.0 rho=1e2

Adding new params to the codebase should not be any different from before, as there is only one copy of the param parser, which is first used on the input file and then used on command line arguments, which gives command line arguments the power to override.

[evaneschneider]

Great idea.