the full path of a BEAST2 example file,
will stop if the filename is not a BEAST2 example file
Arguments
filename
name of the BEAST2 example file. This should exclude
the full path; this function exists to add that full path
beast2_folder
the folder where the BEAST2 is installed.
Note that this is not the folder where the BEAST2 executable
is installed: the BEAST2 executable is in a subfolder.
Use get_default_beast2_folder to get the default BEAST2
folder.
Use get_default_beast2_bin_path to get the full path to
the default BEAST2 executable.