Fix #271: Re-tuned controller for the monopile version

[RRiva]

In #271 Fabio noticed some large vibrations with steady wind above rated. Fabio suggested that increasing the generator speed filter frequency ameliorates this issue. This PR does the following changes to the monopile model:

• Set the frequency of the generator speed filter to 2.5P, as done for the IEA 22 MW. This means 0.315 Hz, which is about double of the previous value.
• Copied all control parameters from HAWCStab2 2.16. In particular, the gains below rated are now higher.
• Added a few more decimals to the minimum and rated rotor speeds.

The improved performance is confirmed by the following parameter study, where I have varied the generator speed filter frequency and computed the std of the pitch angle. As we can see, the old tuning of 0.16 is nearly the worst possible choice.

[jennirinker]

The changes in this file are fine, but I need to add some changes in the monopile and floating htc files to make them consistent.

[jennirinker]

All comments have been addressed in #319 . Closing this PR. Thank you so much for the big effort, Riccardo!