trt_position: Starting and ending position in a vector
Description
Function trt_position() gives the starting and ending
index of covariate values in the i1th group if all the
covariate values from all treatment groups are together in a vector.
E.g., covariate values in group 1 start from 1st value to the
n1 th value; those in group 2 start from n1+1 and end at (n1+n2)th
value.
This function is for retrieving the position of an observations
when the covariate values from all treatments are stored together
in one vector.