Auto-tune the minibatch size

[accosmin]

• the minibatch size has a great influence on performance and it depends on the task at size
• tune 2 parameters: batch size {8T, 16T, 32T, 64T where T = logical cpus} & factor 1 + {0, 1e-2, 1e-3, 1e-4}
• another option is to tune a single parameter: keep the batch size as a parameter, but tune the factor {0, 1e-4, ...., 1e-2}

[accosmin]

solution:

• modify stoch_solver_t to have ::tune() & ::minimize()
• modify stoch_solver_t to store (tuned) parameters as configurable_t::config()
• rename solver files to solver_xxx.cpp/h
• also rename layers to layer_xxx.cpp/h
• move the content of stoch_loop t as private utilities in stoch_solver_t
• modify stoch_trainer_t to tune the minibatch factor by calling ::tune(factors...) in a loop