MF26 multiplicity should always be 1

[whaeck]

The ENDF manual says that the multiplicities must always be 1, not just "normally one". Thus, it doesn't really make sense to require it as an input. I think the only value of this constructor would be to make a user specify the energy range and the law.

That said, maybe consistency with MF6 is desired? So maybe this is okay?

Originally posted by @nathangibson14 in https://github.com/njoy/ENDFtk/pull/143#discussion_r1135806457

Verify if this is the case and add a check to verify this, also see about simplifying the constructor as a result of this.