## Linear Operator-MultiFields bug

The new feature that one can add multifields that do not have exactly the same domain causes trouble with the check_input function of linear operators. One fix would be to allow ANY linear operator to operate on ANY MultiField, which is well defined since a linear operator applied to zero is always zero. Thoughts? (minimal code example for NIFTy crashing below)

```
import nifty5 as ift
space = ift.RGSpace(4)
a = ift.full(space, 1.)
space2 = ift.RGSpace(5)
b = ift.full(space, 1.)
multi = ift.MultiField({'a':a, 'b':b})
var_a = ift.Variable(multi)['a']*2 # Multiplying this by 2 causes NIFTy to crash later
# Here it crashes due to domain mismatch
var_a.gradient.adjoint_times(a)
```