preparation for mapping editor
Early mapping editor

Implements these features:
mapping lines showing the mapping
manipulate the mapping lines by drag and drop
filter values ... filtered rows should be mapped to NaN  (partially)
show mapping of individual data items
Fixed editor not returning new scale correctly

It did work before the previous commit, but I had messed it up and not
checked it before committing
Data item mapping lines, documentation, variables

Fixed issue with data item mapping lines and improved documentation and
variable naming
Revert "Data item mapping lines, documentation, variables"

This reverts commit 3c1a00b0ab9a84a77fe64a2901a12b3ac7638cea.
Revert "Revert "Data item mapping lines, documentation, variables""

This reverts commit 04901bfc45a0c0af98ebb03cc35f09754150f95c.
Merge remote-tracking branch 'remotes/origin/feature_mappingeditor'

Conflicts:
	.gitignore
start integrating the mapping editor
disable mapping editor again til it is working