Axis objects do not at all work well with unequal grids

[Alexander Blinne]

>>> import postpic as pp
>>> a = pp.Axis()
>>> a.grid = [1,3,8]
>>> print(a.grid)
array([ 1.  ,  3.75,  8.  ])

Clearly, the natural thing to expect is, that a.grid returns the same numbers as supplied to its setter.

[Alexander Blinne]

After some discussion, the best way to deal with this, seems to be to implement Axis in such a way that it can store either grid (and extent), grid_node or even both and will only calculate the missing parts. This will enable more flexibility and correct behaviour in both, point and cell-based data.