a character vector, of which the first element
is the command (java, in this case),
and the others are arguments (-jar, in this case, followed
by more arguments.
Arguments
input_filename
the name of a BEAST2 input XML file.
This file usually has an .xml extension.
Use create_temp_input_filename to create a temporary
filename with that extension.
beast2_jar_path
name of the BEAST2 jar file
(usually has a .jar extension).
Use get_default_beast2_jar_path to get
the default BEAST jar file's path