add two-sided or single sided test options

[mps9506]

If null hypothesis = data comes from population with independent realizations and are identically distributed, the alternative hypothesis is that the data follows a monotonic trend. This is calculated with the two tailed test and currently implemented in the script.

However, if the alternative is indeed an "increasing trend" or "decreasing trend" (as currently returned in the script) p values should be calculated with the single tail of the distribution.

See:
Implementation in the trend package in R.

Suggest adding an argument for users to specify the alternative.

Thanks to Moshe Elitzur for identifying this.