Add easy ways to export Py6S parameterisation

[robintw]

This may include:

• Ways of saving the whole SixS object so that it can be loaded again (not just the 6S input file, but the object so that the wavelength can be changed and can pick up the right ground reflectance info)
• Ways of exporting parameterisations easily into a table (eg. LaTeX) to display in a paper
• Other cool things...