Restrictoplus to ElementarySpace

[lkdvos]

Fixes #264

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.85%. Comparing base (5fa3a86) to head (9362910).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/spaces/vectorspaces.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #265   +/-   ##
=======================================
  Coverage   82.85%   82.85%           
=======================================
  Files          44       44           
  Lines        5757     5757           
=======================================
  Hits         4770     4770           
  Misses        987      987           

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