Synchronize field selection with graph depending on selected mode

[rodolakis]

If not Auto-off:

• uncheck a box removes the corresponding curves from the graph (related to Unchecking a checkbox removes the signal from the graph #38)
• remove a curve from the graph uncheck the corresponding curves
• checking a box adds it automatically on the graph
• if a new positioner is checked, automatically update the graph (in both auto-replace and auto-add)

[rodolakis]

With this commit b1196d2:

1. in auto-off:

• nothing happens when selection changes until you push Add or Replace button (intended)

2. in auto-replace:

• checking a new DET replace the graph with the new selection
• changing POS updates the graph
• unchecking all POS default to index and update the graph
  DONE: check index box to visually indicate that POS= index

3. in auto-add:

• checking a new DET adds it to the graph
• DONE with 1e00ccc : checking a new POS updates the selection but nothing happens on the graph; may need to clear first
• DONE with: 1e00ccc: removing all POS does default selection to index but the graph still does not update

TODO: the xlabel and title seem to disappear in some instances. To be investigated. New issue: #76

[rodolakis]

With this commit 1e00ccc the graph gets cleared when:

• the POS changes
• the number of DET decreases

BUT:

• TODO: now we need to track the filePath together with the selection, otherwise we will lose any selection from another file since the selection is updated with the current table view New issue: Track the filePath together with the field selection #77