Support for file-based preprocessing

Currently, the supported way of using MASCOT preprocessing in SPDZ runs the MASCOT preprocessing online on-the-fly and the preprocessed material only ever lives in-memory.

We would like to be able to run preprocessing independent from an online phase and persist it to files, and then consume the preprocessed material later in an online phase. This should hopefully also allow us to use preprocessed material from external preprocessing implementations (e.g., Overdrive).

This task includes the following two high-level subtasks
- [ ] Write preprocessing material to file(s) in some convenient and efficient format
- [ ] Read these files and serve them to the online phase (for SPDZ this involves implementing the `SpdzDataSupplier` interface).