consider changing sig.test parameter

[vsimko]

At the moment, the functions wt.sig, pwtc, wtc, xwt, wt use sig.test parameter.
The significance test is actually implemented inside wt.sig and the other functions only redirect the parameter at some point in their code.

There are three possible values for the sig.test parameter:

• 0 = regular chi^2 test
• 1 = time-average test
• 2 = scale-average test

Then, inside wt.sig, there is a switch (originally if..elseif..else) that computes the significance depending on the parameter. The switch in R is faster than if..elseif..else and it can either use numeric index as a parameter (indexed from 1) or a string value. The problem with numeric indexes is that there is no "default" value possible inside the switch.

It seems to me that it would be beneficial to change the numeric values 0,1,2 into string values such as "regular", "time", "scale" or similar. This would also make the code more understandable.

It would, however, break the current API.
But maybe just upgrading the version solves the problem.