this was a bug that it wasn't using a defined freq attribute if it was
available and a freq argument was not provided at initialization.

Don't use the root logger since it's sometimes used by other modules, and
it's kind of sloppy to do so.

This should fix a bug whereby individual noise calibrations were not being
tracked internally properly.

The calibrations weren't being extracted properly from the noise tuples.

closes #58

Also supports a Budget.calibrations attribute that lists calibrations to be
applied to all noises.

Allows for specifying as a class attribute.

Should cover/simplify the most common use case, equivalent to running:

load()
update(**kwargs)
calc_traces

This patch provides a new nb sub-module that defines classes for managing
and calculating noise budgets.  It provides the following overridable
classes:

nb.Calibration  A noise calibration
nb.Noise        A noise source
nb.Budget       A budget of noises

The Budget class includes a calc_trace() method that will return a traces
dictionary that includes data and trace plot styling for every noise term
in the budget recursively.

The existing included interferometers are updated to define their budgets
using this new interface, and the plot_noises function is updated to
accept the new traces dictionary.

An HDF5_SCHEMA describes how trace dictionaries are encoded into HDF5 files.
The new io module includes functions for writing traces to HDF5 files, and
for reading traces stored in this format.

The command line interface is updated to handle this new structure.