Add basic test cases

- [ ] Create a `data/samples/` directory with a `sample.csv` and `sample.xlsx` file
- [ ] Follow [Hadley Wickham's instructions](http://r-pkgs.had.co.nz/tests.html) to create a test case structure
- [ ] Write a test case for `read_eda` for each file in the `data/samples/` directory. Ensure that the file is read without errors and that the results have the correct number of rows, column and column names
- [ ] Write a test case for `eda_metadata` for each file in the `data/samples/` directory. Compare the results against a pre-defined data structure stored as a separate file in the `data/samples/` directory as `sample.csv.meta.yaml` and `sample.xslx.meta.yaml`. Use the attached `meta.yaml` as a sample
- [ ] Ensure that the tests pass

[meta.zip](https://github.com/gramener/eda/files/1546822/meta.zip)