Add feature in GridVisual to draw border around all pixels and selected pixels

[fabien-colonnier]

added the option showgrid, showselectedpixborder in grid visual and change the showborder option to use flatline as the other functions for consistency.

I added also 2 other examples:

• grid_simple_rand.cpp: it uses random value instead of the sinwave as it shows better the different interpolation method in my opinion
• grid_border.cpp: it shows the new display options with GridVisual. It also shows the limitation as the zaxis is not directly handled and is fixed whatever it is.

The usage of grid as variable name for the border around all the pixels is debatable as it could introduce confusion with Grid class in morphologica. I couldn't come up with better naming but I am open for any suggestion.

[sebjameswml]

Great work!