Enable device_type to be none

[omlins]

This makes the use case of using CPUs only on a system having also GPUs more clear.

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (d6f0413) 3.50% compared to head (04e85b4) 3.49%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
src/init_global_grid.jl	0.00%	4 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff            @@
##           master     #79      +/-   ##
=========================================
- Coverage    3.50%   3.49%   -0.01%     
=========================================
  Files           8       8              
  Lines         657     658       +1     
=========================================
  Hits           23      23              
- Misses        634     635       +1     

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