Add area swept calculation to get_survey_sets

[seananderson]

data$area_swept1 <- data$doorspread_m * (data$speed_mpm * data$duration_min)
data$area_swept2 <- data$tow_length_m * data$doorspread_m
data$area_swept <- ifelse(!is.na(data$area_swept2), data$area_swept2, data$area_swept1)
data <- dplyr::filter(data, !is.na(area_swept))

Note area_km is stratum area! At least add to docs?

[ecophilina]

This has been done in the trials branch. Awaits merge into master branch.