Change 'accumulator' to 'dapol_tree'

[Stentonian]

'accumulator' is from the paper but it is not as descriptive as 'dapol_tree'

In the cli the --accumulator flag could possibly be --type

This seems to mirror the paper best:

struct DapolTree {
  accumulator: Accumulator, // enum with values for ndm_smt, dm_smt, etc
  height: Height,
  master_secret: Secret,
  salt_s: Salt,
  salt_b: Salt,
  max_liability: u64, // this should be used for the range proofs: [0, 2^height * max_liability]
}