Fit GP prior and fix bugs

[mohamed82008]

In this PR, I add the GP prior MLE fit and fix a couple of bugs.

[codecov-commenter]

Codecov Report

Merging #88 (27e302f) into master (838d4ee) will increase coverage by 0.78%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master      #88      +/-   ##
==========================================
+ Coverage   67.63%   68.42%   +0.78%     
==========================================
  Files          36       36              
  Lines        2645     2714      +69     
==========================================
+ Hits         1789     1857      +68     
- Misses        856      857       +1     

Impacted Files	Coverage Δ
src/algorithms/bayesian.jl	96.74% <93.05%> (-1.88%)	⬇️
src/wrappers/ipopt.jl	87.21% <100.00%> (+2.88%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 838d4ee...27e302f. Read the comment docs.