Fix: Strings with a single char in Python

[franzpoeschel]

Follow-up to #1517

Problem: In pipe/__main__.py, dest.set_attribute(key, attr, attr_type) cannot distinguish strings with size 1 from chars (except for explicit char types signed char and unsigned char).

Before #1517, that call converted chars up to string, now it converts strings down to chars.

Old behavior:

  string    /data/600/fields/i_all_energyDensity/axisLabels                         attr   = {"z", "y", "x"}                                                                                                                                                                                                               

New behavior

  char      /data/600/fields/e_all_energyDensity/axisLabels                         attr   = {z, y, x}

Since values such as axisLabels = ["x", "y", "z"] are very frequent, the old behavior was less intrusive.

This PR:

1. Switches back to the old behavior (i.e. cancels out the attempts to cast to char, but keeps the attempts to cast to explicit signed or unsigned char)
2. Makes Attribute::getOptional<>() more flexible in such conversions (Currently openpmd-pipe creates unreadable files)