remove option to write sigma-gamma ratio

[johntruckenbrodt]

In #74 a new sigma-gamma annotation layer (sg) was introduced, which is not defined by CEOS-ARD. In #116 the two annotation options gs and sg were renamed to ratio. The processor since internally decides which mask to write depending on the user input.
Originally, the separation of gs and sg was intended to differentiate between the different sigma naught backscatter conventions ($\sigma^0_T$ and $\sigma^0_E$ respectively). However, since #154, sigma naught is always written as $\sigma^0_T$ and the sigma-gamma ratio becomes $\gamma^0_T / \sigma^0_T$ (from $\gamma^0_T / \sigma^0_E$).
Hence, it would make a lot of sense to entirely remove the sigma-gamma ratio and just write a gamma-sigma ratio in any case because it does not make sense to differentiate between the ratios $\gamma^0_T / \sigma^0_T$ and $\sigma^0_T / \gamma^0_T$. The configuration option should be renamed from ratio to simply gs. Depending on whether the backscatter measurement is $\gamma^0_T$ or $\sigma^0_T$, it would be multiplied or divided by the ratio to retrieve the other convention.