Improve performance of present value functions

[alecloudenback]

of the most basic pv(rate,cfs):

julia> judge(median(new),median(old))
BenchmarkTools.TrialJudgement:
  time:   -98.85% => improvement (5.00% tolerance)
  memory: -98.98% => improvement (1.00% tolerance)

[codecov]

Codecov Report

Merging #52 (209777b) into master (d59fbf9) will increase coverage by 0.12%.
The diff coverage is 100.00%.

❗ Current head 209777b differs from pull request most recent head 7551ba5. Consider uploading reports for the commit 7551ba5 to get more accurate results

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
+ Coverage   96.02%   96.15%   +0.12%     
==========================================
  Files           3        3              
  Lines         151      156       +5     
==========================================
+ Hits          145      150       +5     
  Misses          6        6              

Flag	Coverage Δ
unittests	96.15% <100.00%> (+0.12%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/financial_math.jl	95.12% <100.00%> (+0.20%)	⬆️

---

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 d59fbf9...7551ba5. Read the comment docs.