Fix too optimistic definitions of * and / with scalars

[devmotion]

The definitions of * and / for pd matrices are too optimistic: In general, the resulting matrix is not pd (or psd) anymore, so we should not return a pd matrix type (which currently - at least some of them - do not even check positive definiteness at construction!).

If a pd matrix type is desired, then I guess either one should use a dedicated function or encode in the type domain of the scalar that it is positive (or at least non-negative).

Ref #201.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.26%. Comparing base (61c65b1) to head (1523e1b).

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

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #206      +/-   ##
==========================================
- Coverage   93.37%   93.26%   -0.11%     
==========================================
  Files          11       11              
  Lines         800      802       +2     
==========================================
+ Hits          747      748       +1     
- Misses         53       54       +1     

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