Add check for absolute filename in language model

The issue is that the mert-moses script requires an absolute
path, but that the train-factored-phrase-model script was
happy to write a relative one to the moses.ini file.  This
blocks that behaviour from the very beginning.

scripts/training/train-factored-phrase-model.perl

@@ -3,6 +3,7 @@
 use strict;
 use Getopt::Long "GetOptions";
 use FindBin qw($Bin);
+use File::Spec::Functions;
 use File::Basename;
 
 # Train Factored Phrase Model
@@ -215,6 +216,8 @@
     ($f, $order, $filename, $type) = split /:/, $lm, 4;
     die "ERROR: Wrong format of --lm. Expected: --lm factor:order:filename"
       if $f !~ /^[0-9]+$/ || $order !~ /^[0-9]+$/ || !defined $filename;
+    die "ERROR: Filename is not absolute: $filename"
+      unless file_name_is_absolute $filename;
     die "ERROR: Language model file not found or empty: $filename"
       if ! -s $filename;
     push @___LM, [ $f, $order, $filename, $type ];