Secagg default biprime setup #354

MR description

• unit tests

This MR addresses #354 (closed)

Content:

• add default biprime(s) to component's database when fedbiomed_run xxx configuration create is called. If ALLOW_DEFAULTS_BIPRIME is True, then read list of default biprimes from DEFAULT_BIPRIMES_DIR (else list of default biprimes is empty). Add or update default biprimes to component database; remove from database the default biprimes that are not in the list.
• add setup of biprime component element (triggered by SecaggRequest message). Setup can occur with a default biprime in the database, or with a new biprime computed (currently not implemented, dummy payload)
• use default biprime default_biprime0 (the default biprime distributed with the library) in Experiment()
• add removal of biprime component element from components database. Prevent researcher from deleting a default biprime in nodes' database.
• update secagg tutorial notebook
• extra: add saving of negotiated component in researcher's database both for biprime and server key (needed for biprime, homogeneity plus robustness by adding for server key)
• extra: some refactoring for code factorization (SecaggManager in common, node and researcher *Secagg* can share some methods between server key and biprime)
• fix: for server key and biprime, when re-using a secagg component saved in database, check that the parties lists from saved component is either None (can be used by every party) of a superset of the experiment's parties. This is not a security issue but will cause clear failures instead of timeouts in MP-SPDZ.

You can test with:

# clean configuration
source ./scripts/fedbiomed_environment clean
./scripts/fedbiomed_run  network

# create, configure, launch 2 nodes + 1 researcher

# not needed
# ./scripts/fedbiomed_run node config config-n1.ini configuration create
./scripts/fedbiomed_run node config config-n1.ini -am /data/mvesin/data
./scripts/fedbiomed_run node config config-n1.ini start
# not needed
# ./scripts/fedbiomed_run node config config-n2.ini configuration create
./scripts/fedbiomed_run node config config-n2.ini -am /data/mvesin/data
./scripts/fedbiomed_run node config config-n2.ini start
# not needed
# ./scripts/fedbiomed_run researcher configuration create
./scripts/fedbiomed_run researcher start

# initialize certificates for the 3 components
./scripts/fedbiomed_configure_secagg  node   # or researcher
./scripts/fedbiomed_run certificate-dev-setup

# then test with `notebooks/general-tutorial-secagg.ipynb`

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Guidelines for MR review

General:

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