Add deflection of vertical and gravity errors recipes

[PaulWessel]

THe deflection of the vertical is a standard geodetic measure, and having the gravity errors in the faa (due to oceanography and ice) is useful too. Asked David about desired CPTs but we may need to invent some.

[PaulWessel]

Wel, similar to VGG. They are created arithmetically and does not require an FFT to make gravity anomalies. I think the satellite's orbits are such that deflections are very good in N-S near equator while E-W are strong at high latitude. But, I have never used them for any scientific inquiry. But gmt gravfft, talwani2d and 3d can compute them

[PaulWessel]

Should also say that VGG is the sum of the slopes of the deflection times some constant, so taking derivatives (numerically) introduce short-wavelength noise. Thus, the deflections are the least "polluted" geodes measures.

[Esteban82]

Many thanks Paul!