- Introduce a new D_xy tag which takes x_source and y_source
 - The two sources are attached to a DataConsistentcyGroup. Only If the
   consistency group signalizes a consistent state, the data is updated

It basically decorated the AutoPropertyDescription with the type which
is now in AutpPropertyDescription

Parse "type" attribute for normal properties

Bubble up the type to the parent class

change type of macro pulse number PV to int64_t (as this is the data type expected by DOOCS) and convert if necessary via the TypeChangingDecorator

so far, only scalars work and the meta data (macro pulse number etc. is missing)

Fixes DOOCS server shutdown lock-up

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.

- 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.