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.

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.

Temporary roll-back to the periodic update polling instead of the readAny()-based update logic. This is necessary, since the readAny() implementation seems to be too slow for bigger servers like the LLRF server. A proper solution should be found to improve the performance of readAny() for larger number of variables.