public function sdr_bitfieldColors(node, bitfield) result(colchar)
Return all colors of the given bitfield encoded in an array of characters.
The bitfield has to correspond with the color definition in nodes.
Each character is supposed to hold an ASCII symbol and by setting the
byte bits, we can store up to 7 colors in each character.
Nodes of different colours represent the following:
Solid arrows point from a procedure to one which it calls. Dashed
arrows point from an interface to procedures which implement that interface.
This could include the module procedures in a generic interface or the
implementation in a submodule of an interface in a parent module.