Add tutorial step7 - interpolation

[chiaraMaHe]

Tutorial step 7 for the interpolation process.
Wiki article: https://github.com/DLR-AMR/t8code/wiki/Step-7-Interpolation

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[Davknapp]

When executing the code in debug-mode I get memory-issues (Leftover references). Please have a look at all your allocated resources and check if you free´d or unref´d them correctly.

If I understand the example correctly the element_own_data holds the distance of the element_midpoint to the midpoint ot the sphere. Hence I should see a spherical increase of the element_data in paraview. But I can't observe it, see the attached picture of the initial uniform refined forest.

[Davknapp]

All in all it looks good to me, but when executing I get a refcount error:

[t8] Constructed uniform forest with 4096 global elements.
[t8] Into t8_forest_adapt from 4096 total elements
[t8] Done t8_forest_adapt with 1408 total elements
[t8] Into t8_forest_iterate_replace
[t8] Done t8_forest_iterate_replace
[libsc 0] Abort: Assertion 'forest->rc.refcount > 0'
[libsc 0] Abort: ../t8code/src/t8_forest/t8_forest.c:1676
[libsc 0] Abort: Obtained 9 stack frames
[libsc 0] Stack 0: t8_step7_interpolation() [0x48c969]
[libsc 0] Stack 1: t8_step7_interpolation() [0x48c937]
[libsc 0] Stack 2: t8_step7_interpolation() [0x48cb6c]
[libsc 0] Stack 3: t8_step7_interpolation() [0x4131ed]
[libsc 0] Stack 4: t8_step7_interpolation() [0x405716]
[libsc 0] Stack 5: t8_step7_interpolation() [0x405786]
[libsc 0] Stack 6: t8_step7_interpolation() [0x405827]
[libsc 0] Stack 7: libc.so.6(__libc_start_main+0xf3) [0x7f60974cb083]
[libsc 0] Stack 8: t8_step7_interpolation() [0x404a3e]
Abort(1) on node 0 (rank 0 in comm 0): application called MPI_Abort(MPI_COMM_WORLD, 1) - process 0