Add negative flag

[RobTillaart]

In 0.2.2 the distance table is symmetrical D(x,y) == D(y,x).

with a flag and a few lines extra it can be a magnitude symmetrical and sign opposite table. D(x,y) = -D(y,x)

• if added one must invert the value if (x,y) is swapped.
• if looked up one must invert the value if (x,y) is swapped.
• document the feature
• example
• works only for the whole table.

[RobTillaart]

added in 0.3.0