MAD-X: Call, FILENAME Support

[ax3l]

Many MAD project files use nested files, e.g.:

• https://github.com/slaclab/facet2-lattice/blob/master/MAD/FACET2e.mad8

Supporting the MAD-8

  CALL, FILENAME="other_file"

syntax could be helpful to avoid that we need to linearize such input decks.
http://mad8.web.cern.ch/mad8/user/switch_file.html#call

Seems to be in MAD-9/MAD-X of the syntax

  CALL, FILE="other_file"

http://mad9.web.cern.ch/mad9/user/node69.html
http://madx.web.cern.ch/madx/webguide/manual.html

[ax3l]

Chapter 5: http://madx.web.cern.ch/madx/webguide/manual.html

5.2 CALL

CALL, FILE="filename";

where filename is the name of an input file. The named file is then read until a RETURN statement is encountered, or until the End_Of_File; The file being ”called” may in turn contain any number of CALL statements itself, and so on to any depth.