Add user-controlled logging mechanism for warnings

Sometimes we want to elaborate on error conditions, but SYCL restricts the return type / exceptions thrown. Example: [USM allocation functions](https://github.com/celerity/SimSYCL/commit/d83979854a7225e633844001791bbda19f3ed614) have different reasons for returning `nullptr`.

SimSYCL should have a reporting mechanism that the user can tap into, both for logging and to verify that a certain error condition was met.