[New model] Add Douglass1992

[chrisleaman]

Who is the published the model and in what year?
Douglass (1992)

Please provide a link, doi or citation to the paper
Douglass, S.L., 1992. Estimating Extreme Values of Run‐Up on Beaches. Journal of Waterway, Port, Coastal, and Ocean Engineering 118, 220–224. https://doi.org/10.1061/(ASCE)0733-950X(1992)118:2(220)

If possible, please provide the relevant equation references
For example:

• Eqn 3: Rmax

Any other details that would be useful?
For example:

• This formula gives Rmax, so perhaps we need to introduce a new parameter so it is not confused with R2%.
• The equation uses Hm0 which can be approximated with Hs.