convert to code fails if there are bogus-codes in the hierarchy

[bernhard-da]

converting and re-importing to/from R-code fails when there are bogus-codes in the hierarchy. e.g

t<- hier_create(root = "Total", nodes = LETTERS[1:4])
t<- hier_add(tree = t, root = "A", nodes = "a0")

r <- hier_convert(tree = t, as = "code")
t2<- hier_import(inp = r, from = "code") 

reason: parent nodes are not extracted.

[bernhard-da]

fixed with 82c7b2a