executed on the source or target processes in the put-get operation

*this is required to enable mapping_on_source==false

* this determines whether the mapping is executed at the source or the target component in der put-get-operation

* now the writing of the weight file and the exchange of halo requests is done concurrently
* the writing of the weight file now uses the global ids from the data request for the halo information

* reduces replication of code
* avoids potential race condition

* it reproduces complex configuration described in "Development and performance of a new version of the OASIS coupler, OASIS3-MCT_3.0" (http://www.geosci-model-dev-discuss.net/gmd-2017-64/)

* a component do not need to have transient_grid_refs

adds new field_interpolation_gen constructors yac_new_field_interpolation_gen_src_tgt_2 and yac_new_field_interpolation_gen_src_tgt_mask_2

* old version was based on pointer values, which could have resulted in unwanted results