Reorganize scalars to accept global data of any dimensions

[xroynard]

Problem

• scalars are 0-dimensional global data, but there is no way to include N-dimensional global data such as:
  • vectors (1D)
  • matrices (2D)
  • tensors (ND for any N)

Solution

• store all N-dimensional (for any N>=0) global data, in a single object global_tensors, this object could be stored on disk with one of the following solutions:
  • a single file global_tensors.csv, sorted by dimension, first scalars, then vectors, then matrice...
  • a file for each dimension
    • global_0D.csv
    • global_1D.csv
    • ...
    • global_ND.csv

This changes are major changes, we should be carefull with retrocompatibility, see #97