Optimise initialisation

[chillenzer]

There are two things to do here:

• [x] Only initialise the necessary parts of the memory (which should be a small fraction). The corresponding function already exists but @psychocoderHPC realised in https://github.com/alpaka-group/mallocMC/pull/271#discussion_r1856134360_ that this is not actually used.
• [ ] Do this multi-threaded.

[chillenzer]

The memset that @psychocoderHPC found was completely redundant with the later init call. So, the first item is closed with #271 .