Select frequency in cross table

[kamapu]

Add argument min_freq in crosstable() allowing to filter rare species in the same step.

[kamapu]

This feature is not feasible in this form. Instead, an additional function (e.g. sort_by_freq()) needs to be designed with following parameters:

• formula (used to aggregate and indicating the taxonomic rank.
• data (vegtable object but maybe also a matrix)
• decreasing allowing decreasing sorting of taxa
• logical expression, for instance to select by frequency values

This function won't only calculate frequencies but sort them and select according to the expression.