Add check for required time dimension if a range of times is included

The interp script can optionally take a range of time levels to
interpolate.  Currently, if the expected time dimension is not in the
source file, the script will just interpolate the initial time level
from the source into all time levels of the destination.  This commit
fixes this behavior by aborting with an error in that situation.

landice/mesh_tools_li/interpolate_to_mpasli_grid.py

@@ -221,16 +221,22 @@ def interpolate_field(MPASfieldName):
        if 'time' in inputFile.variables[InputFieldName].dimensions:
            InputField = inputFile.variables[InputFieldName][timelev,:,:]
        else:
+           if timelev>0:
+              sys.exit("Error: --timestart and/or --timeend were specified but the required time dimension of 'time' was not found in the source file.")
            InputField = inputFile.variables[InputFieldName][:,:]
     elif filetype=='mpas':
        if 'Time' in inputFile.variables[InputFieldName].dimensions:
            InputField = inputFile.variables[InputFieldName][timelev,:]
        else:
+           if timelev>0:
+              sys.exit("Error: --timestart and/or --timeend were specified but the required time dimension of 'Time' was not found in the source file.")
            InputField = inputFile.variables[InputFieldName][:]
     elif filetype=='other':
        if 'time' in inputFile.variables[InputFieldName].dimensions:
            InputField = inputFile.variables[InputFieldName][timelev,:,:]
        else:
+           if timelev>0:
+              sys.exit("Error: --timestart and/or --timeend were specified but the required time dimension of 'time' was not found in the source file.")
            InputField = inputFile.variables[InputFieldName][:,:]
 
     print('  Input field  {} min/max: {} {}'.format(InputFieldName, InputField.min(), InputField.max()))
@@ -282,6 +288,8 @@ def interpolate_field_with_layers(MPASfieldName):
        if 'time' in inputFile.variables[InputFieldName].dimensions:
            InputField = inputFile.variables[InputFieldName][timelev,:,:,:]
        else:
+           if timelev>0:
+              sys.exit("Error: --timestart and/or --timeend were specified but the required time dimension of 'time' was not found in the source file.")
            InputField = inputFile.variables[InputFieldName][:,:,:]
        inputVerticalDimSize = InputField.shape[0] # vertical index is the first (since we've eliminated time already)
        layerFieldName = inputFile.variables[InputFieldName].dimensions[1] # second dimension is the vertical one - get the name of it
@@ -290,6 +298,8 @@ def interpolate_field_with_layers(MPASfieldName):
        if 'Time' in inputFile.variables[InputFieldName].dimensions:
            InputField = inputFile.variables[InputFieldName][timelev,:,:]
        else:
+           if timelev>0:
+              sys.exit("Error: --timestart and/or --timeend were specified but the required time dimension of 'Time' was not found in the source file.")
            InputField = inputFile.variables[InputFieldName][:,:]
        inputVerticalDimSize = InputField.shape[1] # vertical index is the second (since we've eliminated time already)
        layerThicknessFractions = inputFile.variables['layerThicknessFractions'][:]