Revision 1419

1. toString returns a cost ref to a string.
2. the return value is a member of the function space.
3. the string member is declared mutable so that toString can lazy init. it.
4. this assumes type_str is virtually non mutable for the life of a FunctionSpace.cpp
5. operator=() is now private and users are declared friend classes; This is temporary.
6. the nullDomainValue is now static const, and it's no longer allowed to have external linkage.
7. as a result of 6, methods of FunctionSpace that need to be exported outside a windows dll need to be individually dll_export/import'ed using the ESCRIPT_..... macro