Fix issue with nested SBM mcmc

This solves a problem when NestedBlockState.mcmc_sweep() is invoked with
nonstandard values of the parameter 'c'.
6 jobs for master in 1150 minutes and 43 seconds (queued for 1 second)
Name Stage Failure
failed
job_gcc_amd64 Test There is an unknown failure, please try again
Expected:
Model evidence for deg_corr = True: -517.408224149 (mean field), -672.911348985 (Bethe)
Model evidence for deg_corr = False: -606.900607981 (mean field), -655.099814808 (Bethe)
Got:
Model evidence for deg_corr = True: -549.845093934 (mean field), -688.382102062 (Bethe)
Model evidence for deg_corr = False: -593.581546241 (mean field), -621.257816805 (Bethe)
bash: line 35: 23217 Segmentation fault (core dumped) python3 /usr/bin/sphinx-build -b doctest . build *.rst demos/inference/inference.rst

ERROR: Build failed: exit status 1
failed
job_clang_amd64 Test There is an unknown failure, please try again
    Degree-corrected DL:          1772.83605254
ln Λ: -47.0510317979
Got:
Non-degree-corrected DL: 1738.00660528
Degree-corrected DL: 1780.01146484
ln Λ: -42.0048595573
bash: line 35: 32635 Segmentation fault (core dumped) python3 /usr/bin/sphinx-build -b doctest . build *.rst demos/inference/inference.rst

ERROR: Build failed: exit status 1
failed
job_gcc_py2_amd64 Test There is an unknown failure, please try again

Document: centrality
--------------------

Encoding error:
'ascii' codec can't decode byte 0xc7 in position 402: ordinal not in range(128)
The full traceback has been saved in /tmp/sphinx-err-bvWUBh.log, if you want to report the issue to the developers.

ERROR: Build failed: exit status 1
failed
job_clang_py2_amd64 Test There is an unknown failure, please try again
67 tests in 1 items.
57 passed and 10 failed.
***Test Failed*** 10 failures.

Document: generation
--------------------

Session terminated, killing shell...
ERROR: Build failed: aborted: terminated
failed
job_gcc_amd64_nosh Test There is an unknown failure, please try again
Expected:
Change in description length: 15.9450145...
Number of accepted vertex moves: 8767
Got:
Change in description length: 6.368298208866599
Number of accepted vertex moves: 3765
bash: line 35: 22270 Segmentation fault (core dumped) python3 /usr/bin/sphinx-build -b doctest . build *.rst demos/inference/inference.rst

ERROR: Build failed: exit status 1
failed
job_clang_amd64_nosh Test There is an unknown failure, please try again
4 tests in 1 items.
4 passed and 0 failed.
Test passed.

Document: inference
-------------------

Session terminated, killing shell...
ERROR: Build failed: aborted: terminated