Defaults for filesplitrand()

Working on updating the documentation and I noticed that we could probably use `fname` as the default for `newbasename` here:

```
filesplitrand(cls,fname,newbasename,ndigs,header=FALSE,sep)
```

To avoid changing order of arguments we'll also need default for ndigs, perhaps 2?

Opening this now so I will remember to come back to it.

Locate filesplitrand() and its documentation, then read the current argument handling and any examples. Confirm the intended defaults for newbasename and ndigs, update the documentation and function signature consistently, and verify that existing argument order remains usable.

Written by the indexing model from the issue text.