String for effect size

[heliosdrm]

Suggested by: Timothy Bates
it would be handy, esp in teaching, to allow directly submitting a string to the effect size parameter of the various functions.

e.g. for pwr.f2.test:

pwr.f2.test(u= 1, f2 = "small", sig.level = 0.05, power = .8)

[tbates]

thanks @heliosdrm !