Make oml_value_copy() always copy strings.
Make oml_value_copy() always copy strings.

- Fixes #259.
- oml_value_copy() would copy the pointer, not the contents,
  in certain conditions, such as if the source string had the
  is_const flag set.  This leads to problems when the is_const
  string is stack-allocated and is then injected into an
  interval filter (since the original stack will be invalid when
  the filter output function is run).
- Changed oml_value_copy() to always copy strings into memory that
  is owned by the destination OmlValue object.

alco90/soml,mytestbed/oml,lees0414/EUproject,alco90/soml,lees0414/EUproject,mytestbed/oml,mytestbed/oml,mytestbed/oml,alco90/soml,lees0414/EUproject,alco90/soml,alco90/soml,mytestbed/oml,lees0414/EUproject,lees0414/EUproject