Major revision: Directory structure improved, sensitivity to width errors improved, modified test case, and simplified MetropolisCalculations.m.

Just afer I released the last version, I found that AirSWOT width
errors should be closer to 5-10 m, rather than 1 m; also, we don't
really know what the width errors will be. Width errors were handled
poorly in the initial version of the source. I've updated to allow for
that, and improved a couple of other things along the way: please use
this version. Here are detailed updates:

1. I modified the directory structure. This really helps when doing
many runs. (I've now done some additional sensitivity tests, and am
more confident that we have more reproducible results).

2. The sensitivity of the cross-sectional area calculations to width
errors were not being handled well; the previous version had a "dA"
uncertainty read in; "dA" represents the cross-sectional area above
baseflow. dA uncertainty is dependent on how you calculate dA from
width and height. I used a simple method for calculating dA, and
implemented an "online" uncertainty calculation of \sigma dA from
\sigma h and \sigma w.

3. The test case is now fairly representative of what we're expecting
from AirSWOT over the Sacramento: three reaches downstream of Colusa,
CA, width errors of 10 m, 10 overpasses.

4. I moved all of the covariance matrix calculations (or at least those
that do not change within the Metropolis loop) into their own
sub-fuunction in order to keep MetropolisCalculations clean.