This is used for spike-ins, whose gtf file does not contain "exon"
features.

The resulting BedTool is now in bed format.

There seems to be a circular dependency issue: updating libhts
downgraded libbamutils, but libbamutils needs newer libhts.

Also have setup.py install the script.

Changed .ix to .loc

Now needs 1.4.2 or above.

Later will have to enable 1.4.2.

It has no default value for older numpy version.