More strict parsing of printing section of inputs file

[MattRolchigo]

• Allow simulation without the "Printing" section of input file present (to avoid printing any simulation data to files other than the log)
• Allow "Printing[PrintIntralayer][PrintIdleFrames]" default of false to be used when this field is not given in the input file
• Check for required inputs in "Printing" section of input file, throwing an error if any are not given
• Check that all inputs given in "Printing" section of input file are valid, throwing an error if an unrecognized field is found
• Fixed typo in examples/Inp_SpotMelt.json