[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