fix of guessChunking to handle unbalanced data sets
Merge pull request #342 from asobolev/master

example to be further developed
As discussed @gnode headquarters there are now optional groups

Optional groups are a functor struct that opens a group in the file if allowed by the user and if it is not open already. Other the ```Group::openGroup``` in the case that it is _not_ allowed and does not exist an unset optional is returned an not an error thrown.
The ```optGroup``` is primed and created by ```Group::openOptGroup```.
So far implemented on ```BlockHDF5::source_group``` only.
Made frontend class-type params be references. Split all multi-getter.

Multi-getter like "references" have now all been split into one without filter param and one with, to avoid default value. This was already done for a subset of multi-getters to allow passing them to validation and thus now is more consistent and allows to use references everywhere (which would not be possible for params with default value).
Merge pull request #343 from balint42/optional_groups

As discussed @gnode headquarters there are now optional groups
Merge branch 'master' of https://github.com/G-Node/nix

Conflicts:
	src/hdf5/DataSet.cpp
remove empty lines, whitespaces
Merge pull request #344 from jgrewe/master

improved stability of guess chunking

lgtm
Implemented name instead of id in HDF5 in case of DataArray
Fixed CreateDataArray using wrong hasDataArray method, added test
Fixed Feature having missleading internal var naming
Made "name" a read-only attribute by removing setter
add support for rad as SI unit
Merge pull request #347 from jgrewe/unit

add support for rad as SI unit
Implemented using name instead of id also for Tag, MultiTag and Source

Also added test for "DuplicateName" error
Implemented use name instead of id for sub Sources

Added test for "DuplicateName" too
Made name a read-only attribute on Property by removing setter

Removed tests too
Implemented use name instead of id for PropertyHDF5

Added test for "DuplicateName" error too
Made Section & Block use name instead of id

Adapted tests too
Added fix to testReopen to avoid duplicate name