Automatically add total_codes for OGD Data

[GregorDeCillia]

The classification files of OGD datasets contain an optional column "FK" (Foreign Key) which can point to the parent element of the clasification element. This allows a definition of hierarchical classifications. Currently, the FK column is ignored by STATcubeR but it could be used to automatically detect "total codes". Example:

Code	Name	FK
WEST	Label for West	TOTAL
EAST	Label for East	TOTAL
TOTAL	Label for Total

Here, there is a singe classification element (Code: TOTAL) and all other elements point to that element via FK. In cases like this, it is reasonable to regard TOTAL as the total code for this classification.

[GregorDeCillia]

STATcubeR now automatically sets the total code if there is exactly one empty entry in the FK column. This issue will be closed as soon as #23 is merged

STATcubeR/R/od_utils.R

Lines 10 to 16 in fb58c48

	od_get_total_code <- function(code, parent) {
	if (sum(is.na(parent)) == 1) {
	code[which(is.na(parent))]
	} else {
	NA_character_
	}
	}