Topic/default/variables as mapping

This MR is for three new features:

• We can now use variables section as dict:

variables:
  var1:
    var2:
      - elem1
      - elem2
  var3:
    var4: hello

old method (list of name/value) still works

• We can now call vars with this way:

#@data/values
test1: <[var3.var4]>
test2: <[var1.var2.0]>

• We can also use inherit for overlay part of dict :

# base1/beaver.yaml
variables:
  test:
    key1: value1
    key2: value2

# base2/beaver.yaml
inherit: ../base1
variables:
  test.key1: value3

result :

test:
  key1: value3
  key2: value2

special thanks to @christophe.devienne for his amazing Job.