Improvement: Rename variables from t to element in t8 scheme default

[lenaploetzke]

Describe your changes here:
Closes #1861
Rename variables from t to element in t8 scheme default. Additionally, this makes the variable naming of some functions in the .hxx and .cxx files consistent.
Hope that i catched all t's.

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[codecov]

Codecov Report

❌ Patch coverage is 90.33079% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.41%. Comparing base (2c5eaa4) to head (f89bccf).
⚠️ Report is 4 commits behind head on 1749-fix-t8_default_scheme-documentation.

Files with missing lines	Patch %	Lines
...8_schemes/t8_default/t8_default_tri/t8_dtri_bits.c	89.47%	30 Missing ⚠️
...s/t8_default/t8_default_prism/t8_default_prism.cxx	50.00%	3 Missing ⚠️
..._default/t8_default_pyramid/t8_default_pyramid.cxx	66.66%	3 Missing ⚠️
...t8_default/t8_default_common/t8_default_common.hxx	0.00%	2 Missing ⚠️

Additional details and impacted files

@@                             Coverage Diff                              @@
##           1749-fix-t8_default_scheme-documentation    #1864      +/-   ##
============================================================================
- Coverage                                     74.41%   74.41%   -0.01%     
============================================================================
  Files                                           101      101              
  Lines                                         19197    19192       -5     
============================================================================
- Hits                                          14286    14282       -4     
+ Misses                                         4911     4910       -1     

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[Davknapp]

Thanks a lot for upgrading the names!