change `julian` argument to `isJulian` throughout

[jonathancallahan]

It turns out there is a base::julian() function. Just to avoid confusion and to communicate that this argument accepts a logical, it should be changed to isJulian.

(In case you don't already know, SublimeText is a wonderful tool for global find-and-replace.)

[lagerratrobe]

$ grep -l "@param julian" *.R | grep -v maiac
getDaylightHours.R
goesaodc_createRasterStack.R
goesaodc_downloadAOD.R
goesaodc_downloadDaytimeAOD.R
goesaodc_listDaytimeFiles.R
goesaodc_listFiles.R

Updated files above and added tests for isJulian param.