#86 show throws on non-interactive backend

[martinjrobins]

Fixes #86

Figure::show() now throws when given a non-interactive backend

was thinking that interactive backends should get their own base class, see #134

[codecov]

Codecov Report

Merging #135 into master will decrease coverage by 1.01%.
The diff coverage is 59.09%.

@@            Coverage Diff             @@
##           master     #135      +/-   ##
==========================================
- Coverage   92.65%   91.63%   -1.02%     
==========================================
  Files          65       65              
  Lines        3389     3442      +53     
==========================================
+ Hits         3140     3154      +14     
- Misses        249      288      +39

Impacted Files	Coverage Δ
src/backend/BackendSVG.hpp	77.77% <ø> (ø)	⬆️
tests/TestFigure.cpp	100% <100%> (ø)	⬆️
src/frontend/Figure.tcc	40% <100%> (-60%)	⬇️
src/backend/BackendSVG.cpp	72.4% <30.76%> (-2.23%)	⬇️

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[codecov]

Codecov Report

Merging #135 into master will decrease coverage by 0.94%.
The diff coverage is 72.72%.

@@            Coverage Diff            @@
##           master    #135      +/-   ##
=========================================
- Coverage   92.65%   91.7%   -0.95%     
=========================================
  Files          65      65              
  Lines        3389    3423      +34     
=========================================
- Hits         3140    3139       -1     
- Misses        249     284      +35

Impacted Files	Coverage Δ
src/backend/BackendSVG.hpp	77.77% <ø> (ø)	⬆️
src/frontend/Data.cpp	98.54% <100%> (-0.16%)	⬇️
tests/TestFigure.cpp	100% <100%> (ø)	⬆️
src/frontend/Figure.tcc	40% <100%> (-60%)	⬇️
src/backend/BackendSVG.cpp	73.55% <52.63%> (-1.08%)	⬇️

---

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[martinjrobins]

the coverage on this one is so poor cause I created a bunch of new functions in BackendSVG that are never meant to be called. Hence #134....