linear slope in onion model

[dirk]

Implement a linear slope for A ==0. Right now A ==0 gives a flat SLD profile given by SLDin, SLDout has no effect.

[butler]

Moving to 4.2 due to lack of resources to achieve in a time and as not critical to 4.1

[gonzalezm]

This is OK, as it agrees with what the documentation says about the parameter A_k:

|A_k| > 0: exponential change between sld_in and sld_out
0 < |A_k| <<< 1 (e.g. A_k = 0.0001): linear change between sld_in and sld_out
A_k = 0: Use constant sld = sld_in for the whole shell. What it could be misleading is that in this case sld_out is not used, but this is clearly indicated in the documentation.

The only small bug is that the 'Show SLD profile' does not represent correctly the last case, as it keeps showing a linear change even when A_k = 0. I am going to fix this.

[ajj]

Closed by PR 40 on sasmodels.

Changeset 587cbcd11accde4629ae70dea4e268ca5893f462