Fire ignition from perimeter may lead to fxlat/fxlong fields in meters

[mefrediani]

Describe the bug
A fire ignited from an input perimeter uses a "polygon" imprinted in the variable LFN_HIST in wrfinput_d0* to start the fire. This is different from a point or line ignition when the fire starts from a single grid point, or a line that are configured on the namelist using fire_ignition_start/end_lon/lat. When the namelist option fire_is_real_perimeter is true, it should not require setting ignition points on the namelist, as those are not used for anything.

However, in the current source code, the coordinates for real or ideal runs (i.e. in degrees or meters) are set based on the values of fire_ignition_start_*. If the user deletes these entries from the namelist, or set them to 0, it sets fire_ignition_longlat=0, which leads to the fire grid coordinates in meters.

L1063 of phys/module_fr_fire_driver.F

    ideal=config_flags%fire_ignition_start_x1 .ne.0. .or. config_flags%fire_ignition_start_y1 .ne. 0.
    real=config_flags%fire_ignition_start_lon1 .ne. 0. .or. config_flags%fire_ignition_start_lat1 .ne. 0.
    if(ideal)call message('Using ideal ignition coordinates, m from the lower left domain corner')
    if(real)call message('Using real ignition coordinates, longitude and latitude')
    if(ideal.and.real)call crash('Only one of the ideal or real coordinates may be given')

    fire_ignition_longlat=0  ! default, if no ignition
...
    if(fire_ignition_longlat .eq. 0)then
       ! ideal
       !  ignition is in m
       unit_fxlong=1.  
       unit_fxlat=1.
       ! will set fire mesh coordinates to uniform mesh below

To Reproduce
Steps to reproduce the behavior:
set these option on the namelist:
&fire
ifire = 0, 2,
fire_is_real_perim = .false.,.true.,
! fire_ignition_start_lon1 = 0, 0,
! fire_ignition_start_lat1 = 0, 0,
! fire_ignition_end_lon1 = 0, 0,
! fire_ignition_end_lat1 = 0, 0,

Expected behavior
The coordinates for the fire refined grid, fxlong and fxlat, should be geographical coordinates not the distance in meters from a point on the domain

Additional context
This can be fixed with a statement that also tests if fire_is_real_perim is true

real = config_flags%fire_ignition_start_lon1  .ne. 0.  .or. &
           config_flags%fire_ignition_start_lat1 .ne. 0.    .or. &
           fire_is_real_perim 

[weiwangncar]

@mefrediani Would you like to make a PR and we can invite @pedro-jm to take a look afterwards?

[mefrediani]

@weiwangncar I can make a PR sometime in Nov-Dec. Is it OK if I bundle a few other minor issues that need to be fixed? For example, the FZ0 calculation is in the wrong place, and we need to allow the hard-coded pseudo-timestep for the level set method to be user-defined.

[weiwangncar]

@mefrediani That would be good making a PR by the end of Dec - earlier would even be better. If these fixes are pertaining to the same module, it should be ok.