General

• Added unit conversions through calcXfactor and calcYfactor routines.

• Added a global SPerrorStack to improve error reporting. Entries are added to the stack using SPreportError and can be retrieved using SPgetErrorStack which returns a string. SPclearErrorStack resets the stack.

• Added modifications to all classes to handle case where user writes an extension of some type into a file which already contains an extension of the same type. Now writes the new extension with an EXTVER keyword set to one more than the highest EXTVER of other extensions of the same type in the file.

• Made checkCompatibility routines consistent across classes and added a check for units.