Type out beam properties

[kenloen]

The current version of the beam properties for the six_x_six is using 2D arrays with a shape of N x 21 for the stiff_matrix and inertial_matrix. I would suggest to type out the components instead similar to what is done for timoschenko_hawc and cp_lambda_beam.

I think the current format is great for reading the beam data into a numpy array, but for manually inspecting the data it is not that easy to find the bending stiffness for the flapwise direction as one have to count the number of columns and remember which column number is correspond to. I think a big part of the windIO formats is it is human readable data, which I think this change would support a more human readable format.

A suggestion for should a change could be:

stiff_matrix:
  ....
  values:
    S11: [..]
    S12: [..]
    ....
    S21: [..]
    S22: [..]
   ....
    S66: [..]
inertia_matrix:
  ....
  values:
    m: [..]
    x_cm: [..]
    y_cm: [..]
    iedge: [..]
    icp: [..]
    iflap: [..]
    iplr: [..]

Where I have assumed that for the inertia_maxtix the matrix is not a full 6x6 which I am not sure is generally true?!

Any comments and suggestions are much welcome.