Optional Mapping configuration

[AlejandroSilvestri]

As for version 0.4.0, all configuration parameters except for Camera node have default values, leading to much smaller configuration files. But stella_vslam fails to run if you entirely omit the Mapping node in yaml file.
It works if you just add "Mapping:", without any actual mapping configuration.
It's a very minimal bug.
This is the line causing the error:

stella_vslam/src/stella_vslam/system.cc

Line 66 in a18add7

mapper_ = new mapping_module(cfg_->yaml_node_["Mapping"], map_db_, bow_db_, bow_vocab_);

I believe it can be fixed by changing the first argument for:

mapper_ = new mapping_module(util::yaml_optional_ref(cfg_->yaml_node_, "Mapping"), map_db_, bow_db_, bow_vocab_);

Not tested, I am sorry I don't pull a request.