Backport isconstant

[jishnub]

Backport changes from #635, #634, #633, #632 and #631

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (16cc267) 71.30% compared to head (e5c6ad8) 71.48%.
Report is 7 commits behind head on backport_0.8.

Files	Patch %	Lines
src/Space.jl	87.50%	1 Missing ⚠️
src/Spaces/DiracSpace.jl	85.71%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           backport_0.8     #637      +/-   ##
================================================
+ Coverage         71.30%   71.48%   +0.18%     
================================================
  Files                81       81              
  Lines              8600     8623      +23     
================================================
+ Hits               6132     6164      +32     
+ Misses             2468     2459       -9     

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