Feature/add day night

[paulf81]

I think ready to be merged

Feature or improvement description
This feature adds a small convenience function that, given the time in a dataframe in UTC and a lat/long coordinate, adds a column to the dataframe is_day that can be used to partition the data into daytime and nighttime. The determination is made using the altitude of the sun calculated via python's ephem package. An option input, lag_hours, allows day/night to start a specified number of hours after sunrise/sunset.

The pull request includes an example using the smarteole data, and a new test.

Impacted areas of the software
dataframe_manipulations.py

[Bartdoekemeijer]

Nice feature!

[misi9170]

@paulf81 I've made a couple of changes, but I think this is good to go. I did add a note to the example notebook indicating that the apparent choice of 0 for the daytime deflection parameter is something we are thinking about, and I've opened a discussion for this here: #142

[paulf81]

Thanks you @misi9170 ! Merging now...