Squashed 'subtrees/dagflow/' changes from 5bd12830..63525278

63525278 feat: bundle load_variable `state` option
b6dc9a47 feat: Parameter/GaussianParameter, `__slots__`, `free`/`constrained`/`fixed`/`variable`/`correlated` properties
289b18f2 feat: load_variables, enable loading the files
4b237e30 feat: more schema modules
249aa720 feat: graphviz add mid-nodes to handle single output to multiple inputs connection
06e5de79 omit subtrees from coverage report
REVERT: 5bd12830 feat: load_variables, enable loading the files
REVERT: 7ace3d4f feat: more schema modules
REVERT: 5932df69 feat: graphviz add mid-nodes to handle single output to multiple inputs connection

git-subtree-dir: subtrees/dagflow
git-subtree-split: 63525278ad079a6aa3ff7b2734c21a67702aca14