Update Standard Name Rules table for dimensionless units, change variables improperly using "1" to "fraction"

[mkavulich]

Related to Issue #26 and discussion therein, this PR updates the Standard Name Rules table for dimensionless units, removing some ambiguity in wording, separating out "frac" from "percent" entries (since fractions and percentages are different things, and can theoretically be converted) and clarifying when "1" should be used. In addition, several standard names that reference fractions are updated to appropriately use "frac" rather than "1".

[nusbaume]

Thanks for tackling this @mkavulich! Would you be willing to add a couple extra unit changes to this PR? Specifically the following:

1. cellular_automata_seed_probability currently has units of fraction. Might be worth changing that to frac.
2. chemical_tracer_scavenging_fractions should probably also have units of frac.

Otherwise I think everything else looks good, at least to me.

[climbfuji]

Why can we not spell out fraction? We also spell out percent. The 8-character limit in F77 is history!

[mkavulich]

@climbfuji I'm just going off the existing conventions; all the schemes in ccpp-physics currently use frac. But if we're okay with committing to that change then I'm all for using fraction.

[nusbaume]

I'm personally fine with either frac or fraction, just as long as we aren't using both of them together. I also agree that fraction is certainly less ambiguous (but would require more effort on this particular PR).

[gold2718]

Looks okay for me

[climbfuji]

If it's not too much work, can you change all frac to fraction please?

[mkavulich]

@nusbaume @climbfuji I have made the suggested changes

[climbfuji]

Thanks for considering my comments!

[nusbaume]

Looks great to me now, thanks!