simplify multiple pulses with common dest deme into a single pulse

[grahamgower]

Now that we support multiple sources in single pulse event, we should simplify to this when possible.

[apragsdale]

Yes - there will be some corner cases that we'll want to be careful about, but this should be straight forward.

By corner cases, I mean the following should not be simplified:

pulses:
- time: 1
  sources: ["a"]
  proportions: [0.1]
  dest: "b"
- time: 1
  sources: ["b"]
  proportions: [0.2]
  dest: "d"
- time: 1
  sources: ["c"]
  proportions: [0.3]
  dest: "b"

but other scenarios can be, e.g.:

pulses:
- time: 1
  sources: ["a"]
  proportions: [0.1]
  dest: "b"
- time: 1
  sources: ["a"]
  proportions: [0.2]
  dest: "d"
- time: 1
  sources: ["c"]
  proportions: [0.3]
  dest: "b"