agroup: build the groups of key-value pairs, grouped by the keys

Usage: agroup -key {expr} -val {expr} [other params]
Parameters:
-key {expr}: specify the expression that calculates the group key
-val {expr}: the previous for the values
-f {file}: specify an input file (instead of the standrad input)
-sep {separator}: specify the separator for the results (between group
  names, values, aggregate results)
-s: single line printing of the grouped values
-v {vardef}: define a runtime variable
-a {expr}: specify an aggregate expression (this is evaluated for each
  group item)
-ar {expr}: specify the aggregate result for the groups (evaluated only
  once for each group, respectively)

Runtime variables:
line: contains the current input line
value: contains the value (specified by the -val parameter) of a given
  record. This value is used to calculate the aggregate results.


Examples:

Let the test file contain the following lines:
apple|10
apple|20
banana|10
apple|40
grape|10
banana|20
apple|50

acat test.txt | agroup -key "strtok(line, '|', 0)" -val "strtok(line, '|', 1)"
  -> the groups are based on the first token of the lines (this the key: the
     name of the group), the values are the second tokens of the lines

     apple|10
     apple|20
     apple|40
     apple|50
     banana|10
     banana|20
     grape|10

... | agroup -key "strtok(line, '|', 0)" -val "strtok(line, '|', 1)" -s
  -> the groups take only one line: each line starts with the group name, then
     it is followed by the values that are assigned to it, separated by the
     (now default) separator ('|')

     apple|10|20|40|50
     banana|10|20
     grape|10

... | agroup -key ... -val ... -v "sum<int>" -a "sum += #value" -ar "sum"
  -> calculate the (aggregated) sum value of the groups

     apple|120
     banana|30
     grape|10

... | agroup -key ... -val ... -v "sum<int>" -a "sum += #value" -ar "sum"
  -v "min<int>=1000" -a "aif(#value < min, min, #value)" -ar "min" -s
  -> calculate the sum and minimum aggregate values for the groups

     apple|120|10
     banana|30|10
     grape|10|10