Changes in version 1.2.0
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Date: 2018-07-13
* change the expression for `model` of `exp3P` using a supscript
* add `trendline_summary()` function
* add `SSexp2P()` function
* add `SSpower2P` function
* add `Pvalue.corrected` argument in `trendline()` and `trendline_summary()` , for P-vlaue calculation for non-linear regression.
* add `Details` in `trendline()` and `trendline_summary()` 
* add `...` argument in `trendline()` as same as those in `plot()`

