1. A method for the automatic optimization of a natural gas transport network in the steady state, the natural gas transport network comprising at one and the same time a set of passive works including pipelines or resistances, and a set of active works comprising regulating valves, isolating valves, compression stations each with at least one compressor, storage or supply devices, consumption devices, elements for bypassing the compression stations and elements for bypassing the regulating valves, the passive works and the active works being linked together by junctions, the optimization method comprising the determination of values for continuous variables such as the pressure and the flow rate of the natural gas at any point of the transport network, and the determination of values for discrete variables such as the startup state of the compressors, the state of opening of the compression stations, the state of opening of the regulating valves, the state of the elements for bypassing the compression stations, the state of the elements for bypassing the regulating valves, the orientation of the compression stations and the orientation of the regulating valves, characterized in that intervals of values for the continuous variables and sets of values for the discrete variables are chosen as initial state of the optimization, in that the possibilities of values for the variables are explored by constructing on the go a tree with branches linked to nodes describing the combinations of values envisaged by using a technique of separation of variables, that is to say of cutting leading to the generation of new nodes in the tree, and of evaluation, that is to say of determination with a high probability of the branches of the tree which may lead to leaves constituting an optimized final solution, so as to traverse by priority these branches having greater probability of success, the values of the quantities sought being considered to be optimal when predetermined constraints are no longer violated or are minimally violated and a predetermined objective function is minimized, this objective function being of the form 
 g=α× Regime+β×Energy+γ×Target with: α, β and γ are weighting coeffecients; regime represents a minimization or maximization factor for the pressure at given points of the network such as any point downstream of a storage or supply device, any point upstream and any point downstream of a compression station or of a regulating valve, and any point upstream of a consumption device, Energy represents a minimization factor for the consumption of compression energy, Target represents a maximization or minimization factor for the flow rate of a stretch of the network situated between two junctions or the pressure of a particular junction, and the said predetermined constraints comprising on the one hand equality constraints comprising the law for the head loss in the pipelines and the node law governing the calculation of networks, and on the other hand inequality constraints comprising minimum and maximum flow rate constraints, minimum and maximum pressure constraints for the active or passive works, compression power constraints for the compression stations.