make memory a parameter

[JavierCladellas]

The number of allocated nodes should also be determined from the application memory, depending on other parameters.

E.g. If the thermal-bridged M3 - P3 case takes 1Tb of memory, and each node has 256Gb and we want to run on 128 tasks, then distribute the tasks on 5 nodes ( tasks per node: 26, 26, 26, 26, 24) .

Example params for [P2-P3] - [M2,M3] cartesian product:

{
"name" : "discretization",
"sequence":[
  {
    "M":2,
    "P":2,
    "mem":"250G"
  },
  {
    "M":2,
    "P":3,
    "mem":"500G"
  },
  {
    "M":3,
    "P":2,
    "mem":"400G"
  },
  {
    "M":3,
    "P":3,
    "mem":"1T"
  },
]
}