Allow `output` and `assert` in the same command

[sswan]

Allow something like

thresh foo.txt cat u1=col2 u2=col1 output gen.csv assert "np.linalg.norm(u1-u2) < 1.0e-6"
thresh foo.txt cat u1=col2 u2=col1 assert "np.linalg.norm(u1-u2) < 1.0e-6" output gen.csv

[eric-c-cyr]

Can we also have output to standard out?

[sswan]

@eric-c-cyr Good point. Printing to stdout would be print and not output. Would you want to have all these options at once (output, print, assert)?

thresh foo.txt cat u1=col2 u2=col1 print .csv output gen.csv assert "np.linalg.norm(u1-u2) < 1.0e-6"

[sswan]

However, print and assert both want to print to the screen. I've been trying to make it so that only one "task" is printing at a time. This could be overcome by making sure that assert is always printing to stderr and print is always to stdout.

@eric-c-cyr What do you think?

[eric-c-cyr]

For my current use case, print would be sufficient. Though since I"m being greedy...yes all at once