Code crashes if 'dist' in Surface tracking becomes NaN

[anuj-git-coding]

Hello!

In case any modification in surface tracking transport operator leads to a 'NaN' flight distance, the code crashes without printing the root cause of the problem (dist = 'Nan'). Perhaps a suitable error message will be helpful for users to debug the code?

Cheers
Anuj

Current crash message ->
Fatal has occurred in:
decreaseLevel (coord_class.f90)

Because:
New nesting: 0 is invalid. Current nesting: 3
<><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
Note: The following floating-point exceptions are signalling: IEEE_INVALID_FLAG IEEE_UNDERFLOW_FLAG IEEE_DENORMAL
ERROR STOP

Error termination. Backtrace:
#0 0x7f64eee86d11 in ???
#1 0x7f64eee87859 in ???
#2 0x7f64eee88f87 in ???
#3 0x5587d6256cab in __errors_mod_MOD_fatalerror
at /home/ad2221/SCONE_ORIGINAL/SCONE/SharedModules/errors_mod.f90:66
#4 0x5587d622400b in __coord_class_MOD_decreaselevel
at /home/ad2221/SCONE_ORIGINAL/SCONE/Geometry/coord_class.f90:318
#5 0x5587d61e7037 in __geometrystd_class_MOD_move_nocache
at /home/ad2221/SCONE_ORIGINAL/SCONE/Geometry/geometryStd_class.f90:238
#6 0x5587d621d73a in __transportoperatorst_class_MOD_surfacetracking
at /home/ad2221/SCONE_ORIGINAL/SCONE/TransportOperator/transportOperatorST_class.f90:199
#7 0x5587d618d4b6 in __transportoperator_inter_MOD_transport.constprop.0
at /home/ad2221/SCONE_ORIGINAL/SCONE/TransportOperator/transportOperator_inter.f90:111
#8 0x5587d6256f9a in __eigenphysicspackage_class_MOD_cycles._omp_fn.1
at /home/ad2221/SCONE_ORIGINAL/SCONE/PhysicsPackages/eigenPhysicsPackage_class.f90:209
#9 0x7f64eead28e5 in ???
#10 0x5587d6257700 in __eigenphysicspackage_class_MOD_cycles
at /home/ad2221/SCONE_ORIGINAL/SCONE/PhysicsPackages/eigenPhysicsPackage_class.f90:198
#11 0x5587d62537fc in __eigenphysicspackage_class_MOD_run
at /home/ad2221/SCONE_ORIGINAL/SCONE/PhysicsPackages/eigenPhysicsPackage_class.f90:130
#12 0x5587d6183954 in scone
at /home/ad2221/SCONE_ORIGINAL/SCONE/Apps/scone.f90:56
#13 0x5587d618352e in main
at /home/ad2221/SCONE_ORIGINAL/SCONE/Apps/scone.f90:3