fix problem in sedimentBottomFlux2D:

* the settling term has also to be divided by H (= bathy+eta)
* the erosion term has to be divided by H instead of bath

use interpolate instead of L2Projection in compute_sediment function of
Tracer class
2 jobs for sed1.0.0 in 37 minutes and 21 seconds (queued for 2 seconds)