Log Message:
Add congen 1.7, from David Flater

congen is a C++ library for generating the speeds, equilibrium arguments,
and node factors of Darwin-style tidal constituents more or less as defined
in SP 98:

  Manual of Harmonic Analysis and Prediction of Tides.  Special Publication
  No. 98, Revised (1940) Edition (reprinted 1958 with corrections;
  reprinted again 1994).  United States Government Printing Office, 1994.

Additionally, libcongen provides limited support for approximating
Doodson-style tidal constituents within the infrastructure of the former.
The Doodson approach is discussed in the following publication:

  Foreman, M.G.G., 1977.  Manual for Tidal Heights Analysis and Prediction.
  Pacific Marine Science Report 77-10, Institute of Ocean Sciences,
  Patricia Bay, Sidney, B.C. (2004 revision).

The Congen header file is intended to be self-documenting with regards to use
of the interface, assuming that one has access to SP 98 and a general
understanding of the subject matter.