Replace "`(_non-null record_)`" with a label connoting size & weight

[benthestatistician]

E.g. "(*mass*)". While at it,

• give examples with nonzero imbal on "(*mass*)" due to:
  • clustering
  • weights
  • null records
• permit NA weights & mark those records as "null".

[benthestatistician]

This wasn't completely well thought out. If we want a balance calculation for mass or size, it's a separate calculation from the "non-null record" row. Revisit whether we want or need such a row later; in the meantime touch up the non-null record row. Closing.