add save figure options to plot_output_var.py script

[tobeycarman]

Currently the plot_output_var.py script does not expose any flexibility to the user for controlling where the resulting image is saved or what the name is. For example:

develop@1a934fa7946f:/work$ plot_output_var.py --yx 0 0 --file /data/workflows/calibration/CMT06-IMNAVIAT/output/cmax_gppallignoringnitrogen/sa-N300/sample_000000000/output/INGPP_yearly_eq.nc
Namespace(annual_grid=False, file='/data/workflows/calibration/CMT06-IMNAVIAT/output/cmax_gppallignoringnitrogen/sa-N300/sample_000000000/output/INGPP_yearly_eq.nc', hide_individual_layers=False, layer_sum=False, layers=[0, 3], pft=None, sharex=False, sharey=False, timesteps=None, yx=[0, 0])
plotting var: INGPP
pixel(Y,X): (0,0)
dim count: 4
dimensions: ['time', 'y', 'x', 'pft']
variables: ['INGPP', 'albers_conical_equal_area']
shape: (200, 10, 10, 10)
selected time range size: 200 start: 0 end: 199

and

develop@1a934fa7946f:/work$ ls *.png
SAMPLE_plot_output_var.png

would be nice to add some control for the user over the location and name of the resulting plot.