LSR2D - Build projection matrix L for straight rays in 2D.
% Input
% Tx: transmitter coordinates, nTx by 2
% 1st column contains X coordinates, 2nd contains Z coordinates
% Rx: receiver coordinates, nRx by 2
% 1st column contains X coordinates, 2nd contains Z coordinates
% grx: vector of X coordinates of cell boundaries (sorted increasing)
% grz: vector of Z coordinates of cell boundaries (sorted increasing)
%
% *** nTx must be equal to nRx, i.e. each row define one Tx-Rx pair ***