Document allowable file types.

[christopherpoole]

The file types and their variants that can be used with CADMesh are poorly documented. In particular, the set of usable file types is different when specifying tetrahedral and tessellated meshes.

Improvements to documentation:

• Indicate that ASCII files are usable only, for most formats binary is not readable.
• When generating tetrahedral meshes, the file type must be explicitly indicated.
• Scientific notation can cause errors, see ASCII STL files with scientific notation #2
• Offer guidelines for exporting meshes for common CAD packages.