"MAX" variables use -spval as missing value, but say label it as +spval

### Brief summary of bug

The variables: T_REF2M_MAX_INST, T_REF2M_MAX_INST_R, T_REF2M_MAX_INST_U set everything to -spval (so that the value will be replaced as it brings in a new max value). But, it labels _FillValue and missing_value as +spval, rather than -spval.

### General bug information

**CTSM version you are using:** release-clm5.0.27

**Does this bug cause significantly incorrect results in the model's science?** No

**Configurations affected:** All

# Details of bug:

This is true in most versions of CLM and CTSM. The lines in TemperatureType.F90 setting to -spval, go back to 2014. But, would have been in place before that.

The only thing that needs to be done is to change the label of the _FillValue/missing_value so that it's truly -1.e36 rather than 1.e36.

### Important output or errors that show the problem

> double T_REF2M_MAX_INST_R(pft) ;
> T_REF2M_MAX_INST_R:long_name = "rural instantaneous daily max of average 2 m height surface air temp (K)" ;
> T_REF2M_MAX_INST_R:units = "K" ;
> T_REF2M_MAX_INST_R:interpinic_flag = 1 ;
> T_REF2M_MAX_INST_R:interpinic_flag_meanings = "1=nearest neighbor, 2=copy directly, 3=skip, 4=area" ;
> T_REF2M_MAX_INST_R:varnames_on_old_files = "T_REF2M_MAX_INST_R" ;
> T_REF2M_MAX_INST_R:_FillValue = 1.e+36 ;
> T_REF2M_MAX_INST_R:missing_value = 1.e+36 ;
> double T_REF2M_MAX_INST_U(pft) ;
>