Fix NaN from missing difficulty of book

[CarsonF]

@atGit2021 did all the hard work of narrowing the NaN down to this function returning undefined.

undefined * x == NaN

I fixed the Psalms name from the name change, added tests to ensure our book map is exhaustive, and am returning 0 as factor when book cannot be found as a last chance safety check.

┆Issue is synchronized with this Monday item by Unito

[atGit2021]

@CarsonF why didn't it pick up the book name since both names, Psalm and Psalms, are listed, just in reverse order?

[CarsonF]

@CarsonF why didn't it pick up the book name since both names, Psalm and Psalms, are listed, just in reverse order?

Those other alternate names are only used when fuzzy matching with Book.named(). This is using book.name which is the always the "canonical" name.

[bryanjnelson]

🚀