extend factor versus character

[pitakakariki]

Should probably treat character variables as factors.

e.g. "a" "b" "c" should extend to "a" "b" "c" "d" "e" "f" not "1" "2" "3" "4" "5" "6".

[pitakakariki]

Related: #187

[pitakakariki]

Annoying if e.g. "a" "b" c" becomes "1" "2" ... "9" "10" since "9" sorts later than "10" giving weird output for powerCurve.

Should also change powerCurve output for character and factor to e.g. "number of ______" since generated extended labels aren't necessarily meaningful to the user.