Tesseroids v1.1: Forward modeling of gravitational fields in spherical coordinates

A collection of command-line programs for modeling the gravitational potential, acceleration, and gradient tensor. Tesseroids supports models and computation grids in Cartesian and spherical coordinates.

Official website and documentation: http://tesseroids.leouieda.com/

This is the 1.1 release of Tesseroids. See the documentation (http://tesseroids.leouieda.com/en/v1.1/) and release notes (http://tesseroids.leouieda.com/en/v1.1/changes.html).

**Changes in version 1.1**:

* The tesseroids license was changed from the GNU GPL to the more permissive BSD license.
* tess2prism has a new flag --flatten to make the prism model by flattening the tesseroids (i.e., 1 degree = 111km) into Cartesian coordinates (so that they can be used with the prismg* programs).
* tessg* programs have a new flag -t used to control the distance-size ratio for the automatic recursive division
  of tesseroids.
* **NEW PROGRAMS** prismpots, prismgs, and prismggts, to calculate the prism effects in spherical coordinates. These programs are compatible with the output of tess2prism.
* **NEW PROGRAM** tesslayers to generate a tesseroid model of a stack of layers from grids of the thickness and density of each layer. tesslayers complements the functionality of tessmodgen
  and can be used to generate crustal models, sedimentary basin models, etc.
* tesseroids now strictly follows the ANSI C standard.
* Bug fix: prismpot, prismgx, prismgy, prismgz, and prismgxy had problems with a log(z + r) when the computation point was bellow the top of the prism (zp > prism.z1). Fixed by calculating on top of the prism when this happens, then changing the sign of the result when needed (only for gz).
* Bug fix: the tessg and prismg family of programs was crashing when the model file is empty. Now they fail with an error message.