Update resample interpolation logic

Summary

df_resample offers the functionality to pass multiple period_delta's. This way different parts of the dataframe will be resampled with a different frequency. But currently the spaces between to points of the different frequency aren't resampled at all.

Also the functionality of df_interpolate is merged back into df_resample (with option missing_data="interpolate")

Explain your context

(Explain what you did.)

Please list breaking changes, new features or bugfixes

• Breaking changes:

  • Restrict missing_data method for df_resample

• New features:

• Bugfixes:

  • Fix unwanted behaviour of df_resample when interpolating
  • Fix resampling with multiple period_delta's

Closes #316