Stop inserting calculated values into layer features

[cudmore]

We do not want to contaminate a users layer with additional features that may conflict with existing features.

Instead, store these calculated values directly in our pandas dataframe (e.g. our tableview model).

We will still grab all pre-existing layer.features and add them to our table. We just won't add any new ones to the layer itself.

Users can still get copies of these calculated columns with copy table.

For example, see:

• _my_layer.pointsLayer._updateFeatures()
• _my_layer.shapesLayer._updateFeatures()

In the above, we assign to the layer.features. This is where we need to store this calculated value internally in _my_layer or the pandas data model?

This is what we need to get rid of:

self._layer.features.loc[selectedList, 'x'] = self._layer.data[selectedList,2]