Add timing information to log footer, including % of time in subproblems

[odow]

Part of #167

Now we get things like this:

  274   -9.43000e+02 -1.04800e+03 -1.15500e+03 -9.03000e+02 -8.38000e+02 -1.35400e+03  4.68961e+00
  276   -9.99000e+02 -1.02800e+03 -1.05900e+03 -9.74000e+02 -8.39000e+02 -1.37600e+03  4.75262e+00
  278   -9.56000e+02 -1.21500e+03 -1.12000e+03 -1.00000e+03 -8.17000e+02 -1.14800e+03  4.76803e+00
  280   -9.22000e+02 -1.14200e+03 -1.30500e+03 -9.63000e+02 -9.35000e+02 -1.20400e+03  4.80084e+00
  282   -9.76000e+02 -1.20000e+03 -1.02000e+03 -8.79000e+02 -8.00000e+02 -1.24100e+03  4.93183e+00
  284   -1.01100e+03 -1.21900e+03 -1.21600e+03 -9.37000e+02 -9.09000e+02 -1.16000e+03  4.93728e+00
  286   -9.59000e+02 -1.03800e+03 -1.01600e+03 -8.92000e+02 -1.08100e+03 -1.25400e+03  4.98020e+00
--------------------------------------------------------------------------------------------------
termination_status: TIME_LIMIT
result_count: 103

Total solve time:          5.01493e+00
Time spent in subproblems: 4.90530e+00 (98%)
Number of subproblems:     288
--------------------------------------------------------------------------------------------------

Which makes it much easier to spot inefficient algorithms

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.92%. Comparing base (da4c7cf) to head (01ecb1f).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##            master     #173      +/-   ##
===========================================
- Coverage   100.00%   99.92%   -0.08%     
===========================================
  Files           12       12              
  Lines         1251     1268      +17     
===========================================
+ Hits          1251     1267      +16     
- Misses           0        1       +1     

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