Add a "part" node, alter semantics of "section"

[Michael]

Part nodes serve as a structural grouping for engineering purposes, as oppose to the logical grouping, done by [section]:

[-->id<
 [general graph goes here]
--]

Notes:

• [call] nodes can only call part nodes.
• When done executing [section] nodes, we always go to their next
• When done executing [--part] nodes, always go back to the next of the call that called the part.
• parts do not contain parts, but can contain anything else.

Parts are out to the normal node flow. So in the following file:

[a]
[b]
[-->xx<
  [x]
  [y]
--]
[c]
[d]

next field of the nodes is:

"main"->[a]->[b]->[c]->[d]
"parts[xx]"->[x]->[y]

Todo:
* [ ] update parser
* [ ] Update runtime and tests
* [ ] Update visualizations and validations 
* [ ] Update docs.

[Mor Vilozni]

c01c00c

Todo:

• update parser
• Update runtime and tests
• Update visualizations and validations
• Update docs.