Add a 'templates' directive.

[Paul-Ferrell]

Templates are like the 'Files' directive (#140), except they refer to a template file that is read, filled in with Pavilion variables, and written out to the specified location.

mytest:
  variables: 
    foo: bar
  run:
    templates:
      'src/Makefile': 'make_template.tmpl'

• The destination file is the key, and the source file is the value to fit better with the syntax for the 'files' directive.
• The destination may be a relative path as per 'files'.
• The entire file should be read as a single PavString, and resolved using the test's variable manager.
  • If this in the the build step, deferred variables are not allowed.
  • If this is in the run step, resolution will have to happen twice just like with the run.sh file.

[Paul-Ferrell]

Maybe Mako or Jinja2?