`Cell.get_ValidCells()` - don't re-calculate, use lookup table

[demisjohn]

Iterating to calculate ValidCells takes a long time (few sec), especially for ~4000 Cells (possibly an unusual case of ~1.5mm die?).

One way to solve this is to calculate the valid cells only when calling a function that affects it, and then store it as an Object property for later lookup - no need to recalculate. For example, the following functions should trigger a recalc:

• Cell/Matrix Shift
• Cell/CellSize
• EdgeClearance changes
• if, during a get_ValidCells() the lookup table is not calc'd, then calculate it.

Apart from the above, get_ValidCells() should just return the lookup table stored int he object (eg. Cell.ValidCells).