microeco v0.7.0

1. add trans_classifier class
2. add CCA method in trans_env class and rename functions associated with ordination.
3. optimize scaled-up methods and parameters in trans_ordination of trans_env class.
4. optimize the format of returned pathway files of cal_tax4fun2 in trans_func class.
5. optimize classes to accept null microtable object when created for external data analysis.
6. optimize text size adjustment parameters in plot_lefse_cladogram of trans_diff class
7. add taxa abundance filtering in trans_diff and trans_classifier object creating
8. add KW_dunn for Dunn's Kruskal-Wallis Multiple Comparisons in trans_alpha$cal_diff and trans_diff$new.
9. optimize KW method in trans_alpha$cal_diff and trans_diff$new.
10. add dendrogram option for clustering based on ggplot2 in plot_cor of trans_env class.
11. add colors instance
12. change use_OTUs_frequency to use_frequency in trans_venn_com function of trans_venn class.
13. fix the bug in trans_func$cal_tax4fun2_FRI()
14. fix the warnings in plot_lefse_cladogram of trans_diff class