Addressing the values of CellVariable

[mhvwerts]

In order to make user code more readable, to adhere more closely to FiPy's conventions, and to be consistent about internal (private) vs user (public) variables/properties, I changed the following property/variable names:

• (old) CellVariable.value becomes (new) CellVariable._value. This refers to the internally used NumPy array containing the values of the cells, including the ghost cells (which should not be touched by user).
• (old) CellVariable.innerCellValues becomes new CellVariable.value. This is the NumPy array containing the values of the cell that are inside the calculation domain (i.e.) CellVariable._value without the ghost cells). It carries the same name and serves the same purpose as FiPy's CellVariable.value. It enables indexed addressing (nice examples can already be found in the Notebooks)
• (old) CellVariable.innerCellVolumes renamed to CellVariable.cellvolume for consistency, and now also a property of MeshStructure

pytest OK.

Version is now 0.3.3.

[mhvwerts]

This had been itching for a while. It was actually not that much work, thanks to a few well-targeted "find and replace all".