Allow using float shape functions

[masterleinad]

Previously, we were not able to compile with float as number type since global_co_shape_gradients/global_shape_gradients/global_shape_values were double arrays.
This pull request fixes the issue by duplicating the storage for float.

[peterrum]

Extending to float is a good idea. However, lets first discuss on the style (especially from the background that we might want in the future multiple shape_values; e.g. different polynomial degrees, over-integration).

[peterrum]

This looks ugly. Any chance to create a struct like:

template<typename T>
struct ShapeData
{
    T global_shape_values[(max_elem_degree + 1) * (max_elem_degree + 1)];
    T global_shape_gradients[(max_elem_degree + 1) * (max_elem_degree + 1)];
    T global_co_shape_gradients[(max_elem_degree + 1) * (max_elem_degree + 1)];
}

In the long term, it would be nice to be able to write __constant__ ShapeData<Number, degree, q_points> or s.th. similar. It would be nice to have as much flexibility as in the case of the CPU version.

[masterleinad]

Not a bad suggestion even if helps only slightly. We still need separate global __constant__ variables (function scope static variables are also not allowed) and this is also the reason that we can't templatize on degree and n_q_points in a meaningful way. These arrays have to exist independent of the polynomial degree. Of course, we could create separate arrays, but that would only waste space.

[masterleinad]

I give up on improving this. Accessing __constant__ memory seems to be really brittle. If you pass a pointer instead of a reference to a C-style array or try to use a struct for structuring the data better, you always get an invalid device symbol error.

[peterrum]

If you pass a pointer instead of a reference to a C-style

I also tried that out some time ago; I had the same conclusion :( Thanks for trying out the struct approach! Let's merge!

[kronbichler]

Nice!

[peterrum]

Thanks!

[masterleinad]

I had to discard the struct again to make it actually work.