Make IsScalarMat always return a boolean

[SoongNoonien]

This fixes #300.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.67%. Comparing base (703d1e1) to head (bd4f4f8).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
gap/projective/classicalnatural.gi	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #390      +/-   ##
==========================================
- Coverage   72.67%   72.67%   -0.01%     
==========================================
  Files          43       43              
  Lines       18400    18397       -3     
==========================================
- Hits        13373    13370       -3     
  Misses       5027     5027              

Files with missing lines	Coverage Δ
gap/base/projective.gi	100.00% <100.00%> (ø)
gap/projective/c3c5.gi	86.81% <100.00%> (ø)
gap/projective/tensor.gi	56.54% <ø> (ø)
gap/projective/classicalnatural.gi	87.87% <0.00%> (ø)

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