[Enhancement] more covariance models

[MuellerSeb]

There are far more covariance models described in literature, than provided by GSTools.
I would suggest adding the following:

• JBessel class (hole effect model valid in arbitrary dimensions)
• simple truncated powerlaw TPL, given by (1-h)^nu valid for nu >= (d+1)/2 (extension to truncated linear)
• HyperSpherical (replacement of the current Intersection model)
• SuperSpherical (HyperSpherical with freely selectable parameter)

More ideas?