Sample names can't end in an underscore

[lweasel]

This breaks the filtering step, where we are using "___" as a separator for block files. We could either change this separator to something more obscure, or warn when sample names end in underscore.

cc @hxin

[hxin]

adding a warning or a sample name check at the beginning might be a quick fix? can also change '___' as I don't think there will be any backwards-compatible issue on this.