Include PSP/EPI-Hi LET in the SEP tools

[jgieseler]

PSP/EPI-Hi LET should be rather similar structured like HET, and general loading already works, so it should for most of the SEP tools be straightforward to include it (LET1 and LET2).

• SEP_Fit-Spectra.ipynb
• SEP_Multi-Instrument-Plot.ipynb
• SEP_Multi-Spacecraft-Plot.ipynb
• SEP_PADs-and-Anisotropy.ipynb
• SEP_PyOnset.ipynb
• SEP_Regression-Onset.ipynb
• SEP_Spatial-Distribution.ipynb
• SEP_Spectra.ipynb