Typos in StFile

The StFile class has a number of typos where the documentation does not match the columns. For example, the correct attribute is I_x, and not Ix.

There is also a problem with the torsional stiffness, which is called K in the HAWC2 manual, and the StFile documentation, but is accessed via I_p. For this we have two options:

• Fix the documentation.
• Cause a breaking change by changing its name to K.

Also, set_value() although straightforward to use is not documented.