Add empty ctor for ViewBroadcast

[bartgol]

Motivation

Sometimes we want to create the object, and defer the initialization until the input view is available, like

ViewBroadcast<to_view_t> bcasted_v;
if (input1d) {
  bcasted_v.setup(input.get<view_1d<T>>(),input.dims());
} else {
  bcasted_v.setup(input.get<view_2d<T>>(),input.dims());
}

with current master, we would have to move the whole code that uses bcasted_v into both the if branches,duplicating everything.

Testing

I modified one bcasted view in the unit tests to be default-constructed, and setup later.