Refactor API (version 1.1); old API remains available.

The API (v1.1) has been updated in order to be more consistent and sane.
The old backward-compatible API (v1.0) remains available in
`coulomb_ho2d_compat.h` (which is automatically included by
`coulomb_ho2d.h`).  The compatibility wrapper `coulomb_ho2d_compat.c`
must be linked in order to use the old API (the makefile does this by
default).

A bug with `resize_arrayd` was discovered and fixed.  This bug does not
appear to have any impact since the `count` is always greater than one
in all practical situations.

The headers have been updated to allow dynamic linking on Windows
systems.  This has not yet been tested.
Add alternative implementation based on OpenFCI and C++ interface

A faster and more accurate implementation based on OpenFCI has been
added.  The makefile can automatically download and build the
dependencies needed by OpenFCI.  The previous implementation (`am`)
remains the default in the makefile.

A C++ interface has been added for convenience.  This interface is a
header-only wrapper around the existing C interface.