Added mirr function in financial_math.jl

[KuldeepBorkar]

Modified IRR which was not included in financial_math.jl is implemented now via present_value

Fixes #61

[alecloudenback]

Please add some new tests to complement the new function (e.g you can probably Google and find a public example, or use Excel and refer to the value it calculates)

[codecov]

Codecov Report

Merging #91 (417e6fa) into master (bf6e13b) will decrease coverage by 3.34%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #91      +/-   ##
==========================================
- Coverage   97.97%   94.63%   -3.35%     
==========================================
  Files           4        4              
  Lines         198      205       +7     
==========================================
  Hits          194      194              
- Misses          4       11       +7     

Flag	Coverage Δ
unittests	94.63% <0.00%> (-3.35%)	⬇️

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Impacted Files	Coverage Δ
src/financial_math.jl	92.99% <0.00%> (-4.34%)	⬇️

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[alecloudenback]

Hello, in addition to the tests one other thing that would be good to do before implementing:

• use accumulation and discount where applicable, which will allow users to flexibly input Yields.jl objects and not just Float64 annual effective rates.

[KuldeepBorkar]

Hello, in addition to the tests one other thing that would be good to do before implementing:

• use accumulation and discount where applicable, which will allow users to flexibly input Yields.jl objects and not just Float64 annual effective rates.

I will try to work on this ASAP.