Now using sqrt(GREAT).

Squashed merge of particleInteractions up to
commit e7cb5bcf0315c359539ef1e715e1d51991343391

Definition in streamLine now redundant.

- use first() method in some places as well where it helps clarity

- there are a few remaining cases: git grep 'size()-1]'

    deltaT().value() to deltaTValue()
and
    deltaT0().value() to deltaT0Value()
across the whole code - faster to return especially if being used
often, in each call to a submodel for example.