More functionality for sparse matrices and rows

[lgoettgens]

Changes:

1. Add can_solve by only doing parts of can_solve_with_solution
2. Adapt Matrix(::SMat) and Array(::SMat) to their docstrings (and move them to the same place). This changed them from returning a AA MatElem to a julia Matrix.
3. Add a similar function Vector(::SRow)
4. Copy the SRow <-> MatElem conversion from Oscar here. This can then be deleted afterwards in Oscar.
5. Allow scalar multiplication from left and from right, and widen signature.

One could consider 2 above breaking, but I would instead call that a bugfix, as the intended behaviour described in the docstring didn't change.
It would be great if I could get a release shortly after merging this to be able to use it in Oscar.LieAlgebras.

[codecov]

Codecov Report

Patch coverage: 82.27% and project coverage change: +0.09% 🎉

Comparison is base (9de3b38) 74.36% compared to head (d9ca7a5) 74.45%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1221      +/-   ##
==========================================
+ Coverage   74.36%   74.45%   +0.09%     
==========================================
  Files         346      346              
  Lines      110757   110806      +49     
==========================================
+ Hits        82361    82504     +143     
+ Misses      28396    28302      -94     

Files Changed	Coverage Δ
src/NumField/NfRel/NfRelNS.jl	84.87% <ø> (+1.34%)	⬆️
src/Sparse/Solve.jl	85.97% <58.82%> (-1.77%)	⬇️
src/Sparse/Row.jl	83.03% <80.00%> (-0.52%)	⬇️
src/Sparse/Matrix.jl	63.98% <100.00%> (+1.66%)	⬆️

... and 22 files with indirect coverage changes

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