GfTool 0.7.0 release

Breaking Changes

• The gftool.pade module had a minor rework.
  The behavior of filters changed. Future breaking changes are to be expected,
  the module is not well-structured.

New Features

• add gftool.lattice.onedim for Green's function of one-dimensional lattice
• add fitting of high-frequency moment to gftool.fourier.iw2tau (e2c92e2)

Other New Features

• add gftool.pade_frequencies (9f492fc)
• add gftool.density_iw function as common interface to calculate occupation
  number from Matsubara or Padé frequencies
• allow calculation of gftool.lattice.bethe for Bethe lattice at complex points
  (note, that this is probably not a physically meaningful quantity) (ccbac7b)
• add stress tensor transformation gftool.lattice.square.stress_trafo for 2D (528fb21)

Bug fixes

• Fix constant in gftool.fourier.tau2iw_ft_lin (e2163e3).
  This error most likely didn't significantly affect any results for a reasonable
  number of tau-points.
• gftool.density should work now with gu-style matrices (4deffdf)

Documentation

• Functions exposed at the top level (gftool) should now properly appear in
  the documentation.