More updates by getting feedback of the users

This version is rather mature after tests.

Get new features from data.table v1.13.0

v0.9.8

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Debug for more robustness

v0.9.7

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Just waiting for updates of data.table

Update vignettes, improve and update two functions.

九五，飞龙在天

Modify the as_dt to suppress the process of making copy, would be much faster.

九四，或跃在渊

Remove in place replacement facilities once and for all.
For that usage, looking forward to the new package tidyft.

Powerful upgrade

v0.9.3

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Super improvement on speed and syntax

v0.8.8

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Enhanced version of tidyfst

1. Optimize group_dt. First, it is faster than before because I use [][] instead of %>%. (Using %>% for .SD is slow) Second, I design an alternative to use .SD directly in group_dt, which might improve the efficiency further.
2. Debug filter_dt.
3. Add fill_na_dt to fill NAs in data.table. Debug all missing functions. Examples are refreshed.
4. Debug mutate_when.
5. Add complete_dt to complete a data.frame like tidyr::complete.
6. Add dummy_dt to get dummy variables from columns.
7. Add t_dt to transpose data frame efficiently.
8. Remove the export of "as.data.table", add two functions:as_dt and in_dt to create a short cut to data.table facilities.

Remove tibble method and improve the select method

v0.6.9

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