Class Profile

Create a new reflectometry 1D profile model.  A profile consists of
four cross sections --- scattering length density rho, absorption
profile mu, magnetic scattering P and magnetic angle theta.

The cross-sections are made up of list layers, all the same length,
or None if that cross section isn't defined.  The top layer and the
bottom layer are semi-infinite slabs.  The middle layers each have
a depth associated with them.  Between each pair of layers is an
interface, with associated roughness to indicate how much diffusion
there is between the layers. This means a system which is made up
of n layers (including incident medium and substrate) has n-1
roughness values and n-2 depth values.

The blend function breaks down if the roughness is too large.  Use
max_rough=k to set k*rough <= depth.  A value of 3 yields a smooth
profile.

Each layer can have a name.  The name can use TeX style math formatting
such as  r'$
m{Si}
m{O}_2' for silicon dioxide.

Various layer types are available.  As of this writing
they include:
    NoLayer    - the profile is not defined for this region
    FlatLayer  - a flat section
    SlopeLayer - a linear section
    SplineLayer - a cubic B-spline section
    JoinLayer   - extend the previous layer with the next depth

Use JoinLayer to create e.g., magnetic profiles which extend
across multple layers and can be fit with splines.

The resulting model has a number of attributes:
    magnetic is True if there are magnetic layers
    absorbing is True if there are absorbing layers
    d is the depth of each layer, including the nominal
        depth of the substrate and incident medium required
        to support roughness (out to 3 sigma)
    offset is the location of each layer interface, including
        values for leading and trailing edge of the incident
        medium and substrate roughness
    rho, mu, P, theta are the layers
    rough is the roughnesses
    names are the names of the layers

Compute the profiles. This uses equidistance steps in z.

TODO: use dz as the criterion rather than n TODO: Collapse excess layers to make computing reflectivity cheaper

Insert a new layer after layer n.

Not implemented.