Inform error message in composite for fhx with no scars

[chguiterman]

If an fhx object (like from this file) contains no fire scars, composite throws an uninformative error:

Error in fix.by(by.x, x) : 'by' must specify a uniquely valid column

This issue should be caught, flagged with a warning, and the composite should return an empty fhx object as is done when there are no events to composite.

[brews]

Closing this issue because it is a duplicate of #131. This should be fixed in master and will be part of the v0.0.5 release.

With the fix in #131, composite() it just returns an empty composite, without warning or message.

Let me know if you have strong feelings otherwise and maybe we can toss a warning() or message() in there. Also comment back here if we're still having this issue.

[brews]

Okay, I'm still getting the error with this file given as an example. I'm removing the duplicate tag and we'll see if we can't fix this for v0.0.5.