memory inefficiency due to initialization

Currently, several structs are created during the initialization. These are then copied into the sys-struct and henceforth only accessed in the latter. The initial structs are still kept in memory, although never used again, leading to a very inefficient RAM usage. Most severely, this concerns the MESH-struct, which for large problems might lead to a huge memory consumption.

Thus, it makes sense to either:

1. avoid the duplicate generation, i.e. directly save the MESH-struct inside the sys-struct. Due to the current setup with the latter, this would cause further modifications in the library.
2. delete the duplicate, original struct after initialization.