Improve GPU support in examples

[victorapm]

• Add HIP support to examples
• Improve examples Makefile (refactoring and GPU support is recognized automatically without the need of passing an extra flag such as use_cuda=1)
• Add custom_ prefix to memory function wrappers (malloc, calloc, and free) to avoid confusion with functions from stdlib.h
• Rename examples ex[1-9] to ex0[1-9] for better folder organization
• Fix compilation warnings
• Add README.md file with a high-level description about the examples and notes about GPU usage

Closes #1129

[victorapm]

Given #268 (comment),

Let me continue working on this. Any feedback is welcomed, e.g., what do you think about the other changes here?

The main reason for including Makefile.config is to get access to how hypre was compiled and info about its dependencies. How could we do that with a plain Makefile that works out of the batch (e.g., without having to pass options like use_cuda=1)? I guess having pkg-config support is a good answer to this