Fix preset values for conventional nuclear input

> Check which preset scenario's have the 3rd gen set to 0.3 plants. There are then two categories:
> > Scenarios with future_year < 2034. These should set the conventional to 0.3 plants (Borssele) and the 3rd gen slider to 0.0

Two presets fell into this category:

* [presets/ec2013/**frans_stokman**.ad](https://github.com/quintel/etsource/blob/a5c4cfa0ef6b5317e57e60f1ef6970ba18558f94/presets/ec2013/frans_stokman.ad)
* [presets/watt_nu/2013/**dick_benschop**.ad](https://github.com/quintel/etsource/blob/a5c4cfa0ef6b5317e57e60f1ef6970ba18558f94/presets/watt_nu/2013/dick_benschop.ad)

> > Scenarios with future_year > 2034. These should set the conventional to 0.0 plants (Borssele) and keep the 3rd gen slider unchanged

This applied to zero presets.

> For the rest: keep whatever explicit slider setting has been created for the 3rd gen plant and set the conventional slider to 0.0

This applies to all other presets.