translation - open datatsets without inverse translation

[bogovicj]

Add infrastructure so that inverse translations are not needed in simple situations.

Specifically - if a provided translation changes the tree structure (paths of nodes in the tree), then an inverse translation should not be required.

• add sourcePath variables to the tree
• check for these variables when reading data
• if sourcePath not present, try inverse translation, if provided.

[bogovicj]

• check for presence of sourcePath
• check for those variables when reading data
• if not there try inverse translation