Simulation() data loading fails if definitions.h doesn't exist

[Simske]

definitions.h is loaded to find the type of coordinate system. It should fall back to the default if the file doesn't exist.

[Simske]

Type of coordinate system could also be loaded from gridfile header, maybe the better option.
Example:

# ******************************************************
# PLUTO 4.3 Grid File
# Generated on  Fri Dec 18 17:17:18 2020
# 
# DIMENSIONS: 2
# GEOMETRY:   POLAR
# X1: [ 0.400000,  2.500000], 256 point(s), 2 ghosts
# X2: [ 0.000000,  6.283185], 768 point(s), 2 ghosts
# ******************************************************

[Simske]

Fixed with above suggestion