Make label loops optional in CtcTreeBuilder for RNA topology support

[larissakl]

As discussed in #99, the difference between the CtcTreeBuilder and a tree builder for Transducer/RNA topology is minimal and a dedicated TransducerTreeBuilder subclass can be avoided.
Therefore I introduced a member labelLoops_ to CtcTreeBuilder, which determines whether self-loops on the non-blank labels are added (CTC topology) or not (RNA topology).
Depends on #98.

[larissakl]

Update:

I introduced the RnaTreeBuilder class as a subclass of CtcTreeBuilder. In SearchSpace.cc, there is now a new config-parameter, allow-label-loop.

• If allow-label-loop is not set, it defaults to true for CtcTreeBuilder ("classic" CTC topology) and false for RnaTreeBuilder ("classic" RNA topology).
• If tree-builder-type = rna and allow-label-loop = yes, the RnaTreeBuilder class is still used, but it will effectively build a tree with CTC topology.
• Conversely, if tree-builder-type = ctc and allow-label-loop = no, the CtcTreeBuilder will build a tree with RNA topology.

I’m still unsure about the code style and the intended behavior, so feel free to share feedback or make any necessary changes directly.

[curufinwe]

Move the allow-label-loop parameter into CtcTreeBuilder and also add it to RnaTreeBuilder, but with a different default value. Then in the RnaTreeBuilder call the parent constructor and afterwards initialize the allowLabelLoop_ member again with the parameter that is inside RnaTreeBuilder.