Scale cores by memory

For some tools like Kraken2 I'd like to define the core count largely in terms of memory, but I also need to use different core counts depending on the destination. Most of the HPC systems I run on allocate memory strictly by core, so you get either 2GB/core or 4GB/core. If I set a static value for cores, I'm wasting cores.

And then occasionally I get allocated full nodes (either if the required memory is > 128 GB or on one particular HPC system that only allocates full nodes), in this case I may have more cores available than I actually want to use due to the slowdowns or increased memory consumption you get with excessively high core counts.

This results in things like [my kraken2 rule](https://github.com/galaxyproject/tpv-shared-database/issues/36#issuecomment-2045870817), the corresponding native spec for the 2GB/core system:

```
submit_native_specification: "--partition=RM-shared --time={time} --nodes=1 --ntasks={int(mem/2)} --mem={int(mem/2)*2000}"
```

where I also override `$GALAXY_SLOTS`:

```
env:
- name: GALAXY_SLOTS
value: "{min(int(mem/2), 64)}"
```

In general for any large memory tool, static core counts are fairly useless, and I may just prefer to always scale cores off of memory in the destination. I'm curious what others think.