Make it possible to run Cholla with particles but no gravity

[mabruzzo]

Overview

This PR makes it possible to run Cholla in a configuration where hydro and particles are enabled while gravity is disabled. This is useful for testing feedback.

A number of small, simple changes were required. These are mostly self-explanatory.

The most important change relates to Particles3D::initialize:

• previously Particles3D::initialize expected Grav3D& as an argument
• since the function only used Grav3D& to access spatial properties, it was relatively easy to change
• I created the new SpatialDomainProps class to be used to convey spatial properties of the grid. It provides a method to create a fresh instance by reading the spatial properties from the main grid object. We now pass this object in place of Grav3D to the Particles3D::initialize
• While I was making these changes, I also modified Grav3D::Initialize to accept SpatialDomainProps as an argument rather than passing all of the details as individual arguments.

Future Ideas

Ultimately, in the future, I think it would be great to make the SpatialDomainProps class take on more importance:

• I think it would be great to directly store instances of SpatialDomainProps directly inside of Grid3D, Particle3D, and Grav3D rather than the current solution where we have 3 different approaches for storing the same data.
• I also think it would be great to create SpatialDomainProps::Get_Position (and have Grid3D::Get_Position forward onto SpatialDomainProps::Get_Position). An added bonus of doing this is we could consider supporting SpatialDomainProps::Get_Position on GPUs (frankly, we might want to do a little more refactoring in that case so we don't need to copy the entirety of SpatialDomainProps on GPUs, but that is very doable)
• Overall, I think this would be a great step towards reducing the complexity of Grid3D, Grav3D, Particle3D, and reducing code duplication