How to merge several integral terms over the cell boundaries in a single integral?

[amartinhuertas]

Whenever we have several integral terms over the cell boundaries in our bilinear form, we cannot merge them into a single integral. (Note that this is possible with other kind of Measures, e.g., when evaluating integrals on the bulk of the cell.) This is so as ArrayBlock does not support entries of different element types. Is reasonable to live with that? Perhaps we could use convert to cope for this, but I am not 100% sure this. I have to think a little bit more how to solve this.