implement count built-ins for DaphneDSL

[m-birke]

sth. to count a specific values like count(matrix[,0], value) (or from a frame)(also on strings)

and/or

unique_counts as know from numpy

There are currently ways to circumvent this by reshaping a matrix into single col, convert it to a frame, register it as a view and finally conduct SQL ops

Or is it the idea to provide such functionality trough SQL?