Count the number of trees in a .trees file
Arguments
- trees_filename
 name of a BEAST2 posterior
.treesfile, as can be read using parse_beast_trees
See also
if the .trees file is invalid,
  use is_trees_file with verbose = TRUE
  for the reason
Count the number of trees in a .trees file
name of a BEAST2 posterior .trees file,
as can be read using parse_beast_trees
if the .trees file is invalid,
  use is_trees_file with verbose = TRUE
  for the reason