New asymmetric laplace-like family

[willwerscheid]

suggested by @stephens999 :

code ebnm_point_laplace to allow asymetric +ve and -ve exponentials

the general case would be 4 parameters: (pi0, pi+,pi-), lambda+, lambda-

a slightly less general case would be (pi0, (1-pi0)/2, (1-pi0)/2)) , lambda+, lambda-

note that this is not the "asymmetric laplace" distribution on Wikipedia... we need a different name