aed_carbon and simExposed issue causing seg fault

[rqthomas]

I was able to diagnose the cause of segmentation fault:

When simExposed == false, the data%id_drytime diagnostic and data%id_atemp environment link are not defined.

libaed-water/src/aed_carbon.F90

Line 350 in 4842d50

data%id_drytime = aed_define_sheet_diag_variable('drytime','d','time cell has been exposed')

libaed-water/src/aed_carbon.F90

Line 400 in 4842d50

IF( data%simExposed ) data%id_atemp = aed_locate_sheet_global('air_temp')

Yet they are used in aed_calculate_riparian_carbon without a check for simExposed == false

https://github.com/AquaticEcoDynamics/libaed-water/blob/4842d506c1fba931333792c63edbca2f8e5d7be5/src/aed_carbon.F90#L957C1-L964C10

and

libaed-water/src/aed_carbon.F90

Line 969 in 4842d50

atemp = _STATE_VAR_S_(data%id_atemp) ! local air temperature

I am happy to do a PR to fix this but I am not sure what behavoir you want. Do you want the diagnostic and environment link to occur regardless of the value for simExposed or do you want the lines in aed_calculate_riparian_carbon to be run only if simExposed == false