Enable Isoform/Unigene switch in cart expression analysis

[nterhoeven]

When analysing carts with the "Expression Analysis" tab, it is not possible to switch between unigenes and isoforms.

According to @iimog this is a design decision. As carts can contain both isoforms and unigenes, it would not be clear how the button works.

I would suggest to do one the following:

• implement two clearly labeled buttons for both funtionalities
• check whether mixed carts are actually used and if not, disable it and enable the switch.

Until this is fixed, users can add both, the isoforms and their corresponding unigenes to a cart.