[io][gridcreator] Make construction of tensorgrid more flexible
- Add a public method taking the positions, cells and the grading vectors as arguments.
- The general makeGrid() method now extracts the necessary parameters from the input file and calls the new method with them
- The new method can be useful in cases where a tensorgrid should be constructed not based on input file data, but on different ones, e.g. evalutated by a user specified algorithm