[GH-160] Add Relative Vigor Index

[jealous]

The Relative Vigor Index (RVI) is a momentum indicator used in technical analysis that measures the strength of a trend by comparing a security's closing price to its trading range while smoothing the results using a simple moving average (SMA).

https://www.investopedia.com/terms/r/relative_vigor_index.asp

Formular

• NUMERATOR= (a+(2×b)+(2×c)+d) / 6
• DENOMINATOR= (e+(2×f)+(2×g)+h) / 6
• RVI= SMA-N of DENOMINATOR / SMA-N of NUMERATOR
• Signal Line = (RVI+(2×i)+(2×j)+k) / 6

where:

• a=Close−Open
• b=Close−Open One Bar Prior to a
• c=Close−Open One Bar Prior to b
• d=Close−Open One Bar Prior to c
• e=High−Low of Bar a
• f=High−Low of Bar b
• g=High−Low of Bar c
• h=High−Low of Bar d
• i=RVI Value One Bar Prior
• j=RVI Value One Bar Prior to i
• k=RVI Value One Bar Prior to j
• N=Minutes/Hours/Days/Weeks/Months

Examples:

• df['rvgi'] retrieves the RVGI line of window 14
• df['rvgis'] retrieves the RVGI signal line of window 14
• df['rvgi_5'] retrieves the RVGI line of window 5
• df['rvgis_5'] retrieves the RVGI signal line of window 5

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.28 :tada:

Comparison is base (abe781d) 97.72% compared to head (b77d722) 98.01%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #161 +/- ## ========================================== + Coverage 97.72% 98.01% +0.28% ========================================== Files 1 1 Lines 880 907 +27 ========================================== + Hits 860 889 +29 + Misses 20 18 -2 ``` | [Impacted Files](https://app.codecov.io/gh/jealous/stockstats/pull/161?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Cedric+Zhuang) | Coverage Δ | | |---|---|---| | [stockstats.py](https://app.codecov.io/gh/jealous/stockstats/pull/161?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Cedric+Zhuang#diff-c3RvY2tzdGF0cy5weQ==) | `98.01% <100.00%> (+0.28%)` | :arrow_up: |

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