Completed test of DSG connections

test_DSG_Connections:  visualizes a DSG and on ON SAC and one OFF SAC.
Can cycle through random ON/OFF points with ENTER.  Where a connection
is established, a vector is drawn to indicate the soma/compartment
heading to visually confirm that it is in an appropriate direction.
ANGLES:  corrected in DSG and Vector2D so that output is bounded between
0 and 359 inclusive.