operator [] method
Returns the value at index, or 0.0 if the position is not stored.
Throws RangeError if index is outside 0..<dimensions.
Implementation
double operator [](int index) {
RangeError.checkValidIndex(index, this, 'index', dimensions);
final i = indices.indexOf(index);
return i == -1 ? 0.0 : values[i];
}