Assembly of BilForms with many terms extremely slow

[paresula]

I am currently working on problems that require me to assemble BilForms with a large number of terms. And even for small numbers of unknowns, the assembly took way longer than I had expected.

This seems to be because in the assembly, the LinearMaps are added by a '+', which results in a tuple, which if I understand correctly then causes long compile times. @sbadrian fixed this once in commit 989f285, but was reverted in commit f8784ed.

Now, I have tried to fix this in PR #134, and for my case it works, and tests are still running, but I feel like I have butchered the code a bit in the process, as there now are if-statements checking whether it's a SpaceTimeBasis also before the return.

If you have any idea how to improve what I did there or how to do it differently, I would be really grateful!