Writing uint8 to the persist file fails

[mhier]

See ChimeraTK/ApplicationCore#154 -> that ticket was reported to the wrong project and belongs here.

[mhier]

In case the conversion functions provided by DeviceAccess are used, this is fixed already. Otherwise change implementation to use those functions (e.g. UserTypeToUserType to convert to std::string).

[mhier]

Now on redmine #8395