Returns an accumulator function which incrementally computes a count, ignoring NaN
values.
accumulator function
var accumulator = incrnancount();
var v = accumulator();
// returns null
v = accumulator( 2.0 );
// returns 1
v = accumulator( -5.0 );
// returns 2
v = accumulator();
// returns 2
If provided a value, returns an updated count; otherwise, returns the current count.
value
count