Add support for older versions of MCNP

[MicahGale]

LANL recently (-ish) posted some archival MCNP manuals. It would be cool to analyze them and add support for these legacy versions.

• MCNPx 2.6.3: https://mcnp.lanl.gov/pdf_files/TechReport_2002_LANL_LA-UR-02-2607_Waters.pdf
• MCNP 2: https://mcnp.lanl.gov/pdf_files/TechReport_1979_LANL_LA-7396-MRev.1_LGT.pdf
• MCNP 1?: https://mcnp.lanl.gov/pdf_files/TechReport_1978_LANL_LA-7396-M_LGT.pdf

[tjlaboss]

MCNP4C: TechReport_2000_LANL_LA-13709-M_Briesmeisterothers.pdf

[MicahGale]

So it seems like we just can't support MCNP 5, and 6.1