Better type checks for differential operator composition

[AlePalu]

Thanks to #2, we can compose differential operators as

auto L = -laplacian<FEM>() + advection<FEM>(b);

which also adds the possibility to compose operators not FE-based. For instance, we could define a 1D diffusion-reaction problem using 1D splines as

auto L = -laplacian<SPLINE>() + reaction<SPLINE>(1.0);

This mechanism opens the possibility to assemble ill-formed operators as -laplacian<FEM>() + reaction<SPLINE>(1.0).
In this case, even if the compilation stops (because of different layouts in the mem_buffer tuple required by FE operators) it would be preferrable to trigger a more understandable static_assert to notify the ill-formness of the operator.