goodpractice alerts

[corybrunson]

I ran goodpractice::gp(), which flagged a few practices that might be changed to make code less fragile (though the quality of the code overall seems solid):

• sapply() can return different structures, especially when simplify is not set to FALSE; vapply() is more stable.
• F and T are variables that can be redefined; FALSE and TRUE are more stable.
• The single-colon operator : can produce unexpected output on edge cases; the seq_*() functions are more stable.

It also indicated that only 65% of code lines are covered by tests.

While i follow these practices myself, i leave it to the developers to decide for themselves; they will not affect my review. : )

Part of this JOSS review.