RAINRATE hack

[JordanLaserGit]

Currently forcingprocessor contains a poor hack to calculate precip_rate for NextGen. RAINRATE should be converted in a physically consistent way.

nwm_variables = [
    "U2D",
    "V2D",
    "LWDOWN",
    "RAINRATE",
    "RAINRATE",
    "T2D",
    "Q2D",
    "PSFC",
    "SWDOWN",
]

global ngen_variables
ngen_variables = [
    "UGRD_10maboveground",
    "VGRD_10maboveground",
    "DLWRF_surface",
    "APCP_surface",
    "precip_rate",  # HACK RAINRATE * 3600
    "TMP_2maboveground",        
    "SPFH_2maboveground",
    "PRES_surface",
    "DSWRF_surface",
] 

[JordanLaserGit]

It looks like we need to implement the code recommended here:
NOAA-OWP/ngen#302
and here:
NOAA-OWP/ngen#509

def rho(temp): 
    """
        Calculate water density at temperature
    """
    return 999.99399 + 0.04216485*temp - 0.007097451*(temp**2) + 0.00003509571*(temp**3) - 9.9037785E-8*(temp**4) 

And

def aorc_as_rate(dataFrame):
    """
        Convert kg/m^2 -> m/s
    """
    if isinstance(dataFrame.index, pd.MultiIndex):
        interval = pd.Series(dataFrame.index.get_level_values(0))
    else:
        interval = pd.Series(dataFrame.index)
    interval = ( interval.shift(-1) - interval ) / np.timedelta64(1, 's')
    interval.index = dataFrame.index
    precip_rate = ( dataFrame['APCP_surface'].shift(-1) / dataFrame['TMP_2maboveground'].apply(rho) ) / interval
    precip_rate.name = 'precip_rate'
    return precip_rate

[jameshalgren]

@chp2001 -- something you could do?
@JoshCu