SPQR: Slightly overhaul code of GPU libraries

[mmuetzel]

This mostly avoids warnings with some compilers. If I understand those warnings correctly, they point to valid "issues":

• Incorrect size of format specifiers when printing to stdout. Avoid that by using a C++ style output stream.
• Missing placement delete operator for overloaded placement new operator. Avoid that issue by using the placement new/delete operators from the STL instead.
• Incomplete template class definition for explicit instantiation of the template class. Avoid that issue by defining the entire template class in a single compilation unit.

[DrTimothyAldenDavis]

It's been a while since I touched this code, and also a long time since I've worked with the C++ placement new technique. It will take me some time to get back up to speed and try it out.

[DrTimothyAldenDavis]

I'd like to converge to a stable SuiteSparse 7.5.0 release soon. I'll try to include these changes.

[DrTimothyAldenDavis]

My GPU tests are passing so I'll merge this in.