update iso_generate_summary_table and support outliers

[sebkopf]

• rename to more generic iso_summarize_data_table
• groups needs to be namespaced
• renamed special char columns don't work, example to reproduce: iso_generate_summary_table(rename(mpg, d 13C/12C = cty), raw=d 13C/12C)
• allow simplified use that takes all columns by default (if no ...) specified expect grouping columns, to generate the data table
• allow flexible functions
• implement outlier identification function based on distance from mean + sd: e.g. outside a 1, 2, 3 sigma range (see nu_tests in isoreader), maybe iso_identify_outliers(n_sigma = 5)
• implement exclude_outliers = TRUE flag (default FALSE) parameter in iso_summarize_data_table that if set checks if outliers column exists in the data table (if not, points to running iso_identifiy_outliers() first), and then calculates the stats only based on those that are not outliers. Will also have to include a col_x n for each column to indicate how many of the total n were used for the calcluation