Small improvements

[steffilazerte]

I forgot to make a pull request before I pushed these changes, but better late than never!

Implemented changes:

• Download data from multiple wells (fixes Download well data from multiple wells simultaneously #14)
• 'Daily' data returns Date not POSIXct (fixes Daily data from get_gwl should return Date not POSIXct #17)
• Warn and skip if well doesn't exist or url is inaccessible (fixes Catch if well number returns no data (incorrect well number) #16)