Making mutable sets explicit

[henryiii]

This steps back from the design from #113, where mutable sets were implicit, and makes them explicit (add_mutable_set, etc.). This way a new user is less likely to be caught with a surprise segfault when they let their set go out of scope after an innocuous add_set. This is a temporary patch for 1.7; 1.8 hopefully will feature a new CLI::Set design that will remove the huge number of different add_set variants. Closes #170.

[codecov]

Codecov Report

Merging #200 into master will decrease coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #200      +/-   ##
==========================================
- Coverage     100%   99.89%   -0.11%     
==========================================
  Files          12       12              
  Lines        1908     1908              
==========================================
- Hits         1908     1906       -2     
- Misses          0        2       +2

Impacted Files	Coverage Δ
include/CLI/App.hpp	99.76% <100%> (-0.24%)	⬇️

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[codecov]

Codecov Report

Merging #200 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #200   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines        1908   1908           
=====================================
  Hits         1908   1908

Impacted Files	Coverage Δ
include/CLI/App.hpp	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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