add possibility to specify layout filename

With MR !25 (merged), in utils.compute_betas_and_lengths the layout file is specified via a flag (LHC or HLLHC). This should be changed to specifying directly the file containing the layout information.

Also, the VELO / DRF / etc. (for HL-LHC) should somehow be included in the layout rather than hard-coded in the compute_betas_and_lengths functions of utils.py. Maybe, as for the materials, to define the layout of all the elements we could have a json on one side, and a python file on the other side (the latter for the stuff for which computations are needed).

See !25 (comment 6870483) and !25 (comment 6870484) .