Input files used by stella

Stella uses three different input files, denoted by the following extensions:

`.in` These are the standard input files that stella uses
`.list` A list of input files that stella can run in parallel, provided enough cores.
`.multi` A list of three input files for use with the multiple-flux-tube radial boundary condition.

The `.in` input file

When stella processes .in files, it recognizes an include directive that takes the form

!include NEW_FILE.in

Using this directive,

Note that the included files can have their own include directive, and so stella handles these directives recursively.

In practice, the !include directive is useful for parameters scans where only one parameter