Implementation of position to index methods

new methods to get the index belonging to a position in a Dimension. This is part of issue #84. There are two things to discuss here:

* Shall we really keep this in an extra util file? IMHO these methods could well go to the respective Dimensions, they are so closely related to the dimension itself. I know that there is a school, saying they should be kept separate but if I were the user I would look for them in the dimensions. 

*   The behavior of the positionToIndex for RangeDimensions: at the moment the method accepts positions that are not in the list of ticks but within the range covered by the dimension. On the other hand positions beyond the extremes of the range lead to an out of bounds exception.