linear to log spectral axis conversion

Is there the ability to convert a Spectrum1D object's spectral axis from linear to log space (and reverse), with flux conservation? I have seen other issues (e.g. #255) discussing resampling/rebinning with or without flux conservation, but I haven't seen this particular case discussed. There are probably numerous implementations of this case out there (e.g. Michele Cappellari's ppxf package has a function called log_rebin - [online docs here](http://www-astro.physics.ox.ac.uk/~mxc/software/#ppxf)), would be great to have this integrated into specutils. Its a common step in a lot of analysis that involved radial velocities.

Start by reviewing issue #255 and the referenced ppxf log_rebin documentation to understand how resampling and flux conservation are handled. Define the expected behavior for converting a Spectrum1D spectral axis between linear and logarithmic spacing in both directions. Done means the capability is integrated into specutils with flux conservation and documented or tested behavior.

Written by the indexing model from the issue text.