Inverse Percentiles of a sample.

USAGE
p = INVPRCTILE(x,xq)
p = INVPRCTILE(x,xq,dim)
p = INVPRCTILE(x,xq,dim,plot_pos)

INPUT:
x - Vector or Matrix of sample data
q - Values for non-exceedance probabilities to be computed.
q can be scalar or vector
dim - Dimension for matrix to be worked for non-exceedance probability
plot_pos - plotting positions that determine interpolation method

OUTPUT:
p - Non-exceedance probabilities values for q.

When x is a vector, p is the same size as xq, and p(i) contains the non-exceedance probability for xq(i) value.

When x is a matrix, the i-th row of p contains
the non-exceedance probability for xq(i)-values of each column of x.

For N-D arrays, INVPRCTILE operates along the first non-singleton dimension.

EXAMPLES:
x = rand(100,1);
q = [0.1 0.25 0.8];
p = invprctile(x,q);
% Check with prctile to get back the same results
qvalues = prctile(x,p)