Add alpha diversity calculation lesson

Author: HediaTnani

episodes/alpha-div.md

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+---
+title: "Calculate alpha diversity"
+teaching: 0 # teaching time in minutes
+exercises: 0 # exercise time in minutes
+---
+
+:::::::::::::::::::::::::::::::::::::: questions 
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+- How to calculate alpha diversity?
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+::::::::::::::::::::::::::::::::::::::::::::::::
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+::::::::::::::::::::::::::::::::::::: objectives
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+- Estimate alpha diversity indices (Shannon, Faith's, evenness and richness)
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+## Introduction
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+This is a lesson created via The Carpentries Workbench. 
+::::::::::::::::::::::::::::::::::::: keypoints 
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+- Use `addAlpha` function to calculate the alpha diversity
+
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+[r-markdown]: https://rmarkdown.rstudio.com/