Ensure TimeAxis w/o orientation fails cleanly

Fix minor bug where if TimeAxis is instantiated w/o an orientation, it fails
in a hard-to-decipher way because the orientation.toLowerCase() breaks on
input before the AbstractAxis class has a chance to throw a proper error.
Just a warning here being that

this goes back to what I was commenting before that it is dangerous to call instance methods in constructors, and this could be one such instance where this._isHorizontal assumes that this.orientation is set before the call is made.