Pad zeros

[mayer79]

This PR implements the new default that some features with zero interaction will be shown in pairwise/threeway stats (in line with pairwise_m, threeway_m).

• The behaviour can be changed in the statistics functions by setting zero = FALSE.
• This change allows the workflow:
  1. Select m most important features v via perm_importance()
  2. Run e.g. s <- hstats(..., v = v, ..., pairwise_m = length(v), threeway_m = 0)
  3. Get all pairwise interaction statistics via h2_pairwise(s), including also features without interactions.