gpsgridder eigenvalue reader

**Description of the desired feature**
For now, gpsgridder is able to output eigenvalues to a file, but yet no option to read that in for later analysis, e.g., changing the number of eigenvalues used. Would be great if there is an option to read that file in, as re-computing eigenvalues takes a lot of time.

**Are you willing to help implement and maintain this feature?** Yes/No