Update writing of tendency terms to avoid plotting errors

[grantfirl]

• conditionally write tendency terms to output file (only if the index in the big tendency array is >= 1, i.e. used) -- this replaces the previous function of always initializing the tendency variables in the output file whether they were used or not in the run, resulting in arrays of missing values
• the scm_analysis.py script was modified to treat these variables as optional variables in the output; if they're not present for a given output file, they're filled in with zeros (not NaNs or -999) because they are often plotted together with other tendencies, and it is better to see 0 tendencies than -999 or something
• reinstate conv_tendencies plot for tutorial

[dustinswales]

@grantfirl Looks great! Just a small typo fix