note on policy for "open" structures

see two previous changesets.

(partially) closed Ctree works with Test_Isac

Notes:
# without this specific "open Ctree" (and with "! activate for Test_Isac", as usual not commited)
e.g. "fun locatetac" always takes the 1st definition (i.e. "end-of-calculation")
# test/../open-structure.sml demonstrates this behaviour TODO

--- closed structure Ctree

Notes:
# "closed" means removed "open Ctree" immediately after respective end.
# test do not work (with "! activate for Test_Isac") due to 1 "open Ctree,
see next changeset.

added structure Ctree : CALC_TREEE

Note: and opened it immediatly,
so only the signature had to be extended for tests.

cleaned code in ctree.sml

Note: a first trial to add structure Ctree caused hang-up in Build_Thydata,
so adding is done in steps; this is the first one.

added structure Solve INTERMEDIATELY

Notes: see 1d3ef477d9c8. This also means, that
# the respective code is not cleaned
# no signature is available, etc.

added structure Applicable INTERMEDIATELY

Note: introduction of structure Ctree : CALC_TREE is in the works,
but this is persuasive such, that "open Ctree" seems useful,
but the latter seems to require a structure to localise it.

renamed structure to Generate : GENERATE_CALC_TREE

Note: only few funs shall go to structure Ctree

cleaned signature MODEL_SPECIFY

Note: comments (* for ... *) shall be removed after cleanup of structures

added structure Specify : MODEL_SPECIFY