initialize batches, if compiled with '-DINIT_BATCH'

Description

The mpi_debug builders frequently fail with floating exceptions. The reason seems to be that due to vectorization memory beyond nst is accessed (although never used later on). In order to make the debug builders happy, I initialize the whole array (incl. padding) when compiled with a special define (-DINIT_BATCH).

News snippet

initiliaze batches to zero when compiling with -DINIT_BATCH.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.