Basis set interpolation of ADC amplitude vectors

[mfherbst]

A very cool feature would be to be able to switch basis sets and use an interpolated previous solution as a guess. This requires roughly the following steps:

• Obtain the overlap matrices in old and new basis
• Run the SCF in the old and new basis
• Import both SCFs into the adcc/libtensor framework
• Build an object performing the interpolation small => large.
  This is the tricky bit, since one needs to be able to tell libtensor about the
  dimensionality (and symmetry) of this matrix.
• Interpolate
• Use as guess for new run