add chi2 around Balmer lines and include the number of degrees of freedom

[moustakas]

• [ ] Do not divide {LINENAME}_CHI2 by the number of degrees of freedom.
• [ ] Report the number of degrees of freedom corresponding to RCHI2_LINE and RCHI.
• [ ] Return the chi2 (and the dof) centered on the Balmer lines.

[moustakas]

• [x] Do not divide {LINENAME}_CHI2 by the number of degrees of freedom.

Done in https://github.com/desihub/fastspecfit/pull/137/commits/d2aca63d7334ebf1b32ecf863181ceeb63085d35.

[moustakas]

Done in #142.