New ICGEM format with piecewise-linear cofficients for gravity fields is not supported

Some recent gravity fields published by ICGEM use a new file format.
A typical example is Eigen 6S2 from http://icgem.gfz-potsdam.de/tom\_longtime
(the original Eigen 6S format is supported). The file reference a link to
the format that is broken. However, the file format seems easy to understand.

The main difference with the previous version (2011) of the file format is
that the time-dependent coefficients now have associated time spans, and
several spans can be defined for the same coefficient.

This means the model is not linear plus harmonic, but rather piecewise-linear
plus harmonics.

The J2 coefficient in the Eigen 6S2 gravity field has 29 different intervals,
from 1950 to 2050, some intervals being one-year long while others are several
decades long.

(from redmine: issue id 448, created on 2018-05-14)