calculateLWDSiteLength: Calculate length of area sampled for LWD
In jasonelaw/nrsa: Calculate metrics for NRSA protocol data.

calculateLWDSiteLength is used to calculate the length of the sampled area so that counts/length and counts/area can be calculated. For wadeable, sites the length of stream where lwd is sampled is just the total reach length for the site, because the lwd plots are the entire area between the transects. For non-wadeable sites, the site length is the number of sampled transects * 20, because each littoral plot is 20 m long.

1	calculateLWDSiteLength(reachlen, numtran, is.wadeable)

`reachlen`	a vector of reachlength metrics for the sites. For wadeable, sites use calculateWadeableReachLength. For Non-wadeable sites, use calculateBoatableReachLength.
`numtran`	a vector with the number of transects sampled. Use observedLWDTransects, to determine the number of lwd transects measured.
`is.wadeable`	a logical vector; `TRUE` if the site was wadeable