[GH-152] Extract Kaufman's Adaptive Moving Average

[jealous]

The Efficiency Ratio (ER) is calculated by dividing the price change over a period by the absolute sum of the price movements that occurred to achieve that change.

The resulting ratio ranges between 0 and 1 with higher values representing a more efficient or trending market.

The default column is close.

The default window is 10.

Formular:

• window_change = ABS(close - close[n])
• last_change = ABS(close-close[1])
• volatility = moving sum of last_change in n
• KER = window_change / volatility

Examples:

• df['ker'] retrieves the 10 periods KER of the close price
• df['high_5_ker'] retrieves 5 periods KER of the high price

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.44 :tada:

Comparison is base (2bac991) 98.26% compared to head (01bc7bf) 98.71%.

:exclamation: Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #153 +/- ## ========================================== + Coverage 98.26% 98.71% +0.44% ========================================== Files 1 1 Lines 922 931 +9 ========================================== + Hits 906 919 +13 + Misses 16 12 -4 ``` | [Impacted Files](https://app.codecov.io/gh/jealous/stockstats/pull/153?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/153?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Cedric+Zhuang#diff-c3RvY2tzdGF0cy5weQ==) | `98.71% <100.00%> (+0.44%)` | :arrow_up: |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.