Added persistency for D_spectrum properties. This resolves #11 partially only, since there is no option to switch it off. I am not sure if that is needed though.

Issue:
Server throws when there are no device module variables:

terminate called after throwing an instance of 'std::logic_error'
  what():  ReadAnyGroup has no element with AccessMode::wait_for_new_data.
Aborted (core dumped)

This commit introduces a workaround as proposed by jens:
  void DoocsUpdater::updateLoop(){
       if (_elementsToRead.empty()) {
           return;
       }

This commit is intended to be reverted if the above strategy has side
effects.

- update dependency versions

- Replace the std::reference_wrapper<TransferElement> with a TransferElementAbstractor.

Fixed a type confusion between int64_t (aka long int on 64 bit platforms) and long long int. The types are not identical (in the C++ sense) but bit-compatible. ChimeraTK expects int64_t while DOOCS expects long long int.

add enable_latest_cxx_support.cmake and added a comment to enable_cxx11_support.cmake to no longer use is.

Ignore configuration files for Visual Studio Code.

fixed another uncought exception when converting e.g. an Inf from double to float (was still occurring for D_spectrum)

Do not stop server (even through assert -> undefined behaviour without Debug flags!) when trying to create a string array. Instead the property is simply ignored and a warning is printed.