Add validate-sfile, a checker for SW4 velocity models

[lispandfound]

Adds an sfile validator, which runs a number of checks over an SW4 velocity model, namely:

• All required attributes and groups exist, and the declared ngrid count matches the number of supplied grids.
• The types of the datasets match what SW4 is expecting to read. This is more important that it looks because SW4 inherits the C-style coding culture of never checking anything. So, if a double is read into a float memory buffer, it will silently write into garbage and stack smash.
• NaN and inf checks for all arrays.
• All interfaces between mesh refinements are checked to ensure they are in increasing order of depth and don't clip each other, or that the topographic gradients are not too high.
• Check that the material density is greater than zero.
• Check Vp, Vs, Qp, Qs > 0 and Vp/Vs >= 1.
• That each grid is at least two gridpoints thick.
• That the grid corners of the velocity model align with geographic coordinates.