If the indicator function returns an xts object or a vector of the same length 
as mktdata, the columns created by the indicator function will be added to the 
mktdata object via cbind.
If the indicator function returns multiple columns, the label will be 
paste'd to the end of either the returned column names or the 
respective column number when applying it to mktdata.
If the indicator returns some more complexobject, it will be added to a list 
in the $indicators slot inside the applyStrategy execution frame.
If you want your indicators to return a more complex object, 
(such as a model specification and output from a regression), be advised
that your signal generator, and potentially your rule function, will need
to understand, anticipate, and know how to manipulate the internal strategy frame.