Recording more detailed memory consumption metrics

[JanVogelsang]

Currently, the VMSize kernel value for the process running NEST can be accessed via the memory_thisjob sli-command (see processes.sli line 1165). However, other metrics like the VMPeak value (peak memory consumption) and VMRSS (resident set size; actual physical allocated memory) are equally important and should be read out.
As we are moving away from SLI, I implemented the functionality in Python (see below), however I am unsure about the correct place to put the code. Could someone of the PyNEST team create a PR out of the code I provided?

def _VmB(VmKey):
    _proc_status = '/proc/%d/status' % os.getpid()
    _scale = {'kB': 1024.0, 'mB': 1024.0*1024.0, 'KB': 1024.0, 'MB': 1024.0*1024.0}
    # get pseudo file  /proc/<pid>/status
    try:
        t = open(_proc_status)
        v = t.read()
        t.close()
    except:
        return 0.0  # non-Linux?
    # get VmKey line e.g. 'VmRSS:  9999  kB\n ...'
    i = v.index(VmKey)
    v = v[i:].split(None, 3)  # whitespace
    if len(v) < 3:
        return 0.0  # invalid format?
    # convert Vm value to bytes
    return float(v[1]) * _scale[v[2]]


def get_vmsize():
    """Return memory usage in bytes."""
    return _VmB('VmSize:')


def get_rss():
    """Return resident memory usage in bytes."""
    return _VmB('VmRSS:')


def get_vmpeak():
    """Return peak memory usage in bytes."""
    return _VmB('VmPeak:')