Assure l1 threshold propagates to json

With this MR I tried to make sure that the L1 thresholds are passed all the way down to JSON through the menu code. We need this to configure a different threshold than is used to build the item.

Summary of the changes:

Now, there seemed to be a confusion about the L1 item and L1 threshold throughout the menu code. The part that is relevant for sequences is the threshold and I believe ChainParts should be using L1 thresholds and should be naming it L1threshold rather than L1item. Tagging @stelzer & @bernius on that.

There is fragment of code in ChainDict creation that I was not able to understand. Commented out now. I believe what it may do is that: for L1N* seeded chains it contracts the list of thresholds to be be of size 1. @bernius can you confirm? I'll make comment in this place.

The functionality from the menu to parse chains/L1 items is used now in CF so everywhere the menu naming convention has to be applied. I.e. _L1X postfix added to the chains names.

Respective test references had to be updated.

In slice fragments there was a piece listing chain info. This was a bit of boiler plate given that the Chain class has repr method. Used that one.

There is a preparation to remove entirely the L1 item from the Chain constructor. In fact only the threshold is useful in this situation. BTW. The L1 item will be part of each chain name.