set default getter for basis.data.index.count
ability to set dataset as first argument for index constructors

BEFORE

dataset.getIndex(basis.data.index.count());
dataset.getIndex(basis.data.index.sum('amount', 'amountChanged'));

AFTER (alternative)

basis.data.index.count(dataset);
basis.data.index.sum(dataset, 'amount', 'amountChanged');
new index-calc preset percentOfSum
simplify setting value for footer
simplify code