CRAIG backward error

[dpo]

This PR

• closes CRAIG should stop based on the backward error #119 by implementing a stopping test based on the backward error
• implements a stopping test for ill-conditioned operators
• implements an option to transfer to the LSQR point at the end of the iterations
• removes old useless semicolons from the code.

It's easiest to examine each commit separately.

[amontoison]

The error with SmallNumber is linked with :
JuliaLang/julia#29662
Maybe we shoud drop julia 0.7?
With julia 1.1, julia 0.7 is not maintained anymore.

[coveralls]

Coverage decreased (-0.1%) to 81.362% when pulling 7c78091 on craig-bkward into 9d8d898 on master.

[dpo]

Yes, let's drop 0.7!

[amontoison]

Done in #122

[codecov]

Codecov Report

Merging #121 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #121   +/-   ##
=======================================
  Coverage   81.56%   81.56%           
=======================================
  Files          21       21           
  Lines        1774     1774           
=======================================
  Hits         1447     1447           
  Misses        327      327

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[codecov]

Codecov Report

Merging #121 into master will decrease coverage by 0.16%.
The diff coverage is 77.63%.

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
- Coverage   87.03%   86.87%   -0.17%     
==========================================
  Files          21       21              
  Lines        1774     1798      +24     
==========================================
+ Hits         1544     1562      +18     
- Misses        230      236       +6

Impacted Files	Coverage Δ
src/craig.jl	83.33% <77.63%> (-2.57%)	⬇️

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