Facederivatives

Fixing face derivative calculations for collapsed elements. The previous face calculations only worked for tricubic-Hermite elements. Some examples, e.g. linear Lagrange-bicubic Hermite, will change as things are now done correctly. Fixed exporting of sector elements. The code no longer crashes but it still doesn't produce .ex files that can be read into cmgui. 
Would be nice to have comments saying where these numbers come from, eg. `/* 6 = FIELD_DATA_POINT_BASED_INTERPOLATION */` or maybe just "See FIELD_IO_EXPORT_ELEMENTAL_GROUP_HEADER_FORTRAN for meanings of interpType constants."

Is there not an easy way to just use the constants from Fortran directly?