Add particular systems for convenience

[prisae]

Add particular systems, e.g.,

• DUALEM (work of @mariacarrizo)
• WalkTEM (example in gallery, work with @leonfoks)
• TEMFAST (work of @aignerlukas)

in a new file systems.py. With this we should also address #137 .

Also, create more TEM examples for the gallery, as many questions come in this regard. E.g.,

• #211 (For apparent resistivity equations see Spies and Frischknecht, Electromagnetic Sounding, Page 297, Table 2, in Vol 2 of Electromagnetic Methods in Applied Geophysics.)