Add option to use more complex python code as parameter value

[JanVogelsang]

Currently, JUBE only allows parameters with the mode set to python to be simple expressions, which is quite restrictive.
This PR adds the possibility for any arbitrary python code to be passed as a parameter value, which will then use the code's return value as parameter value.

[thobreuer]

Can you share a few parameter examples that use this extension?

[JanVogelsang]

I just have a single concrete example where I use this functionality in my benchmarks (manually generating a cpu-bind-mask to prevent the cpu pinning bug on the JSC systems to affect our results):

- name: cpu_bind_mask
        type: string
        separator: ;
        mode: python
        "_": |
           exec("cpu_bind_masks = list()\nbits_per_task = $threads_per_task\ncurrent_block = 0\nnum_blocks = 0\ncurrent_block_free = 4\nfor _ in range($tasks_per_node):\n    bits_left = bits_per_task\n    cpu_bind_masks.append('0x')\n    while bits_left >= current_block_free:\n        mask = '1'*current_block_free\n        cpu_bind_masks[-1] += hex(int(mask, 2))[2:].upper()\n        bits_left -= current_block_free\n        current_block_free = 4\n        num_blocks += 1\n    if bits_left > 0:\n        mask = '1'*bits_left + '0'*(4-bits_left)\n        cpu_bind_masks[-1] += hex(int(mask, 2))[2:].upper()\n        current_block_free -= bits_left\n    cpu_bind_masks[-1] += '0'*current_block\n    current_block += num_blocks");return ",".join(cpu_bind_masks)

However, this is equivalent to any parameter that can't be written in a single line, e.g.:

- {name: scale, type: int, mode: python, "_": "import math; math.sqrt($num_nodes)"

Ideally, if one could just define custom functions that one could then call inside parameter value definitions (i.e., eval calls), there would be no need for exec anymore. Using exec is just a simple workaround that I could implement myself without digging too deep into the Jube code.