better test for monotonically increasing/decreasing

[jordansread]

for

rLakeAnalyzer/R/center.buoyancy.R

Line 32 in 2253bd9

if (depths[2] - depths[1] < 0 ){stop('depths must be in descending order')}

we only test the top two depths, then ignore anything below but have an assumption that depths aren't scrambled below the top two.

So this will make it through the test:

depths = c(0, 0.5, 1, 9.5, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 
         17, 18, 19, 20)

we can use

all(depths == cummax(depths))

to test for this if we need depths to be monotonically increasing (or cummin(depths) for decreasing)

[lawinslow]

Weird, that isn't a very useful check.

Alternative to all(depths == cummax(depths)), could use is.unsorted to check. Big question here really is do we want to sort it for the user, or just throw an error?