Automate BBH simulations

[nilsvu]

Continuing discussion from #2987.

Remaining tasks are:

• Continue ringdown from the last inspiral segment. Probably first by running a new executable. We can either continue the segments or run in a separate ringdown directory.
• Sort out directory structure for eccentricity reduction and initial data. This should remain quite flexible (scripts shouldn't make any assumptions about this directory structure).

[nilsvu]

Discussion with @nilsdeppe @markscheel @wthrowe:

• We converged on a concept of "pipelines" for BBH, BNS, BHNS, supernovae, etc. A pipeline is essentially an automation driver that runs after a simulation has finished and calls into other spectre Python modules to do the actual work (like scheduling simulations, archiving, etc).
• We might call a step in a pipeline an "epoch" (or just "step" or "stage"?).
• The pipeline continues epochs, segments, levs, etc. It uses Python modules to share logic between different pipelines (eccentricity reduction, etc). The pipeline is responsible for choosing directory names etc.

The directory structure for a BNS pipelines might look like this:

# Ecc reduction
Epoch_001_Id/  # (with or without suffixes)
Epoch_002_Ev/  # Runs at Lev_01
Epoch_003_Id/
Epoch_003_Ev/  # Runs at Lev_05
# Start all levs from end of Epoch_003 (PBandJ)
# Start at t = t_pbandj > 0 (don't relabel to t=0)
Epoch_004_Ev/Lev_01...04
    Lev_01/Segment_0000
         Segment_0001
         # ...
         # Switch to single BH
         Segment_0045
    Lev_02/Segment_0000
         Segment_0001
         # ...
         # Switch to single NS
         Segment_0048
         # ...
         # Switch to single BH
         Segment_0089
    # Or organize with stages when switching executables:
    Lev_02/
        Stage0/
            Segment_0000
            Segment_0001
            # ...
        # Switch to single NS
        Stage1/
            Segment_0000
            # ...
        # Switch to single BH
        Stage2/
            Segment_0000
Epoch_005_AssembleForArchive/Lev*
# Maybe a higher-level tool is responsible for the rest of the archiving:
Epoch_006_Archive/
Epoch_007_PushToCatalog/

• When a simulation ends with exit code 0 the submit script calls the pipeline specified in the input file metadata:

# Metadata in input files:
Next: spectre.Pipelines.Bns