These times scales may be used in SP3-c files.

The support is compliant with ISO-8601 standard.

Fixes issue #237

Users can now tune max degree, max eccentricity power and max frequency
in true longitude for short period terms.

The global DSSTCentralBody force model, which was only a container
for one zonal model and one tesseral model, has been deprecated.

This will allow in a later step finer tuning of both zonal and tesseral
terms. If we had kept DSSTCentralBody, the construction parameters would
have been a huge mess of maxIndices (indeed, even separating zonal from
tesseral will induce constructors with numerous parameters ...)

Fixes issue #236

Thanks to Espen Bjørntvedt for the translations

Thanks to Thomas Neidhart for the translations!

Thanks to Beatriz Salazar García for the translations