fix mixing lines keyword args bug

[AndrewLoeppky]

Description Of Changes

Hello! Im an atmospheric science grad student at UBC. I found + squished a bug in the skewt mixing lines function -- if you specify both pressure and mixing_ratio as not the defualts it raises an error. solution is to transpose the input mixing_ratio array as with the defualt.

Critiques/comments welcome, this is my first edit/pr to a code base that isnt mine...

Checklist

• [ ] Closes #xxxx
• [ ] Tests added
• [ ] Fully documented

[CLAassistant]

All committers have signed the CLA.

[dopplershift]

Congratulations on your first merge PR to MetPy! :tada: We hope we see you around more in the future!