Seamount density

[PaulWessel]

Description of proposed changes

This PR adds new capabilities to grdseamount and grdflexure. These allows us to compute a variable seamount density structure informed from the dozen of so seamounts that have been imaged seismically. There are three new options:

1. -HH/rho_l/rho_h[+ddensify][+ppower]. This defines the rho(r,z) function for a seamount with reference to a standard large seamount of height H and various parameters.
2. -Kdensmodel writes out a crossection grid of the normalized reference seamount, mostly for use to illustrate the variable density function (see figure below).
3. -Wrhogrid will write out one or more vertically-averaged density grids that go with the one or more seamount grids produced. These can be used in flexure calculations as variable load densities.

The documentation has been updated and a new figure has been added:

grdflexure has received a few changes as well in order to handle variable density structures:

1. The -D option accepts a load density as -, which means it is variable.
2. A new -Hdensgrid passes the vertically-averaged density grid (or grids via a template, like for the topography).

Both man pages will see more updates with examples once these are fully worked out and a paper that is in works sees the light of day [it will be Open Access which can then be linked to].

Because these are custom research tools in a supplement I would like these approved and merged asap.