QOL Enhancements to MultiView

[rchen20]

Is your feature request related to a problem? Please describe.

Describe the solution you'd like

@gberg617 requested a few enhancements to MultiView:

• Ability to take a const data_type ** as data in the MultiView constructor. @gberg617 will provide an example use case.
• Ability to create an empty MultiView without passing in data.
• Ability to get the Layout from a MultiView.

Describe alternatives you've considered

Additional context

[gberg617]

@rchen20 A simple example for const data_type ** is shown below:

  const double value = 0.0;
  const double* value_ptr = &value;
  const double** value_ptr_ptr = &value_ptr;
  auto mv = MultiView<const double, Layout<1>>(value_ptr_ptr);

This currently produces an invalid conversion error when trying to cast a const double ** to a double ** in the constructor of the MultiView.

[rchen20]

Suggestion: Instead of empty MultiView, maybe just pass void * or void **. This is more consistent with normal RAJA Views.

[MrBurmark]

It makes sense to me to be able to default construct a view and have it set to nullptr.

[rhornung67]

This should be addressed by this PR: #1866