New logo!

• stplanr now has a logo! See #334
• line_to_points() depreciated in favour of od2line(), the latter function name being more consistent with the package's other functions
• line2pointsn() now works with sf objects
• Documentation fixes - see #329

OTHER

• Various improvements made to the stplanr-od vignette, thanks to Edward Leigh
• URLs updated to link to stplanr's new, official website: https://docs.ropensci.org/stplanr/