Rework Budgeting

[sjperkins]

Closes #55

The idea here is that we should establish a basic number of visibilities that exercise a CPU core for, say a second, and use that as a baseline for memory budgeting across the cores on a CPU, rather than trying to make the number of visibilities in a chunk as big as possible.

• Tests added / passed

  $ py.test --flake8 -v -s .

  If the flake8 tests fail, the quickest way to correct
  this is to run autopep8 and then flake8
  to fix the remaining issues.

  $ pip install -U autopep8 flake8
  $ autopep8 -r -i .
  $ flake8 .