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
beast2_path
name of either a BEAST2 binary file
(usually simply beast)
or a BEAST2 jar file
(usually has a .jar extension).
Use get_default_beast2_bin_path to get
the default BEAST binary file's path
Use get_default_beast2_jar_path to get
the default BEAST jar file's path