Change the way we calculate 2d levels

Please use the branch 'levels' for this purpose.

We have currently the following options:

wheel and shiftWheel should change the corresponding contourLevels[0] parameter

The only way to change the contourLevels[1] if using the slider in the spectrum option panel.

We precalculate once for ever the allowed: https://github.com/cheminfo/nmr-displayer/blob/c8793c1fc279fd98572b031d741f33b6e31f00de/src/data/data2d/Processing2D.js#L44-L45

When we want to calculate the _range in https://github.com/cheminfo/nmr-displayer/blob/c8793c1fc279fd98572b031d741f33b6e31f00de/src/data/data2d/Processing2D.js#L138-L143 we use the values from this.allowedLevels and the 'numberOfLayers' between contourLevels[0] and contourLevels[1]. if there is not enough levels we limit the number of displayed levels.

Once this is done and because now only a limited number of levels is allowed we can cache the result of the 101 possible level values so that we don't need to recalculate them each time.