Simplify the creation of new fields

[rodja]

It would be great if the field creation process could be further streamlined. We need:

• a field boundary as GeoPoint list to ensure the robot stays in it's geo fence
• rows with start and end GeoPoints which define the lines where the robot should use it's implement
• the crop which is cultivated on the field (which then can be used to optically follow the row)
• optional list of other plants which should be kept

[rodja]

I think it's ok for now if we assume:

• all rows are straight
• a field has not more than 1 hectar (eg. manual driving along is no problem)
• the row spacing and crop spacing is even

With this I think we could build a nice wizzard with the following basic flow:

1. User clicks on "Create Field" in the Field Planner.
2. The creation wizzard opens in a dialog.
3. The first step shows a live feed of the front facing camera with a vertical blue line in the middle and below it the message: "Please drive the robot to the leftmost row of the new field, right before the first crop, so that the blue line runs over the row".
4. After confirming, the operator must provide the cultivated crop and it's spacing.
5. Then, the operator is asked to press a button so the robot starts following the crops until user stops and thereby confirms end of row.
6. Now the augmented live feed front camera is shown again with the message: "Please drive the robot to the last row on this side, right before the first crop, so that the blue line runs over the row".
7. After confirming, the robot starts driving down the last row and should stop automatically at the end (because the geometry of the field is fully known).
8. If the operator confirms the geometry, the last required input is the number of rows in the field (or the row spacing).

We could also estimate the crop spacing and most likely cultivated crop in later additions.

[rodja]

I have implemented the idea in #75.