Computes the sample size required for a 1-group sign test with desired power (see size.test.sign.ps for a paired-samples sign test). The Sign test is a test of the null hypothesis that the population median is equal to some specified value. This null hypothesis can also be expressed in terms of the proportion of scores in the population that are greater than the hypothesized population median value. Under the null hypothesis, the population proportion is equal to .5. This function requires a planning value of the population proportion.