fix threading issue with SetField/GetField
fix threading issue with SetField/GetField

SetField and GetField were modifying the corresponding TIFF function arguments "on-the-fly".
This means that if two threads were calling these functions simultaneously (with tags of different type),
the call would fail due to TypeError.
=> Just use the dynamic support of ctypes to pass the arguments.
As long as the arguments are the right c_type, the Right Thing will happen.

brianhelba/pylibtiff,brianhelba/pylibtiff,brianhelba/pylibtiff