Better define public and private interfaces of BlockSeries

The correct way of using BlockSeries is:

1. You are allowed to modify data or .eval
2. You may only access terms by using .evaluated

Someone may be tempted to use BlockSeries.data while forgetting that the data may change in the future, or call .eval, which avoids caching.

We should make this explicit by:

• Renaming data → _data
• Making .eval a property with a setter but no getter.