'rude downsampling of a `CMADataLogger` data file by `factor`, keeping
also the first `first` entries. This function is a stump and subject
to future changes.
Arguments
- `factor` -- downsampling factor
- `first` -- keep first `first` entries
- `switch` -- switch the new logger name to oldname+\'down\'
Details
``self.name_prefix+\'down\'`` files are written
Example
import cma
cma.downsampling()  # takes outcmaes* files
cma.plot(\'outcmaesdown\')'

def downsampling(self, factor=10, first=3, switch=True):

    newprefix = (self.name_prefix + 'down')
    for name in CMADataLogger.names:
        f = open(((newprefix + name) + '.dat'), 'w')
        iline = 0
        cwritten = 0
        for line in open(((self.name_prefix + name) + '.dat')):
            if ((iline < first) or ((iline % factor) == 0)):
                f.write(line)
                cwritten += 1
            iline += 1
        f.close()
        print(((((('%d' % cwritten) + '    lines    written    in    ') + newprefix) + name) + '.dat'))
    if switch:
        self.name_prefix += 'down'
    return self
