Add verbosity for differentialTest and suggest `slam`

[nickp60]

Hi corncob maintainers! Would this be a useful addition? I find myself wondering when things are going to finish processing, and thought this verbosity flag might be useful.

Additionally, running the example code from the documentation from differentialTest in a clean R environment yielded the All models failed to converge! error, but with the added verbosity it appears that was due to the lack of the slam package. I didn't see it listed in the DESCRIPTION so I added it to the suggests section.

[codecov-commenter]

Codecov Report

Merging #125 (4473ab5) into master (7f146f2) will decrease coverage by 0.04%. The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
- Coverage   89.31%   89.26%   -0.05%     
==========================================
  Files          30       30              
  Lines         973      978       +5     
==========================================
+ Hits          869      873       +4     
- Misses        104      105       +1     

Impacted Files	Coverage Δ
R/differentialTest.R	97.27% <80.00%> (-0.83%)	:arrow_down:

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[nickp60]

@bryandmartin any thoughts on this? It looks like the checks are warning about Ghostscript being missing in the test environment, and the error is noting that warnings were thrown.

[bryandmartin]

Hi @nickp60,

Sorry it has taken me so long to get around to this! I had to put corncob aside for a bit in order to take care of some other things. However, this looks good and I will be merging the request! Thank you for your patience.

[nickp60]

No worries, thanks @bryandmartin ! Hope its helpful for folks...