Various code cleanups.

Add corresponding unit test.
Closes #66

Use OpmResult defined in error module.

Add general OpmResult typedef to error module.

Further code cleanup

code cleanup

This cleans up the example code a little bit.
Fixed detector.export_data.
Generate diagram before performing analysis. If analysis fails,
a diagram is generated anyway.
Update various examples to use the aboe save function.

opticscnery example now exports model file (opticscenery.opm)

Edge connections still missing.

nodes and edges are read but grpah is not yet constructed.

Fix clippy warnings.

No node factory method yet