Update help and examples to cover point and line transects

[techisdead]

The actual maths underpinning this package is equally applicable to point and line transects, using Distance package methods to calculate the effective detection radius, or effective detection (half) width respectively. In the development and vignettes we have leaned more on point transect examples so the line transect usage is not clear to the average punter.

So we need to

• confirm for ourselves that the observed_flux works in both cases (double check)
• check and update the help entries for the observed_flux and wrapper functions
• do a search for all references to effective detection radius and make more general
• add words to the deterministic vignette (maybe a little sub vignette) to make the calcullation of the flux really obvious
• check and tests still pass.
• submit for PR