Concatenate or combine two fhx objects
# S3 method for fhx
+(a, b)An fhx object.
The fhx object to be append.
An fhx object with the observations from a and b.
Throws stop() if there are duplicate series names in a and b.
series_names() get all the series in an fhx object.
get_series() subset an fhx object to select series.
delete() remove observations from an fhx object.
sort.fhx() sort an fhx object.