Use correct field number of components in xi and gauss point interpolation functions

These were assuming the interpolated field number of components is equal
to the number of geometric dimensions, but any kind of field with an
arbitrary number of components could be used.
Fix Python Field.ParameterSetGetGaussPoint wrapper method
Merge pull request #182 from adamreeve/xi_interpolation

Use correct field number of components in xi and gauss point interpolation functions