Document / measure resource consumption

[gakonst]

We should document how many resources are required, depending on:

1. ceremony size
2. batch size
3. number of threads the program is run with (easily configurable via the RAYON_NUM_THREADS env var)

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Preliminary notes

In each batch we load:

batch * (3 * g1 + g2) elements. Assuming g2 = 2*g1 in size, this means we will hold 5 * g1 * batch per concurrent thread on each batch.

In the contribution, the received input is uncompressed, while the output is compressed, so g1_in = g1_out * 2.

in_batch = 5 * g1_in * batch_size_size
out_batch= 5 * g1_out * batch
total_per_batch = in_batch + out_batch = 15 * g1 * batch_size

If we're on BLS12, g1 = 48.

So memory_per_batch ~= 720 * batch_size bytes