Windows support

[adamkemberling]

Many of the functions that find resources on box rely on either:
box_path() or its complimentary cs_path()

Currently, anything downstream of the main folder, things that are appended to make the full path using str_c(), are not respecting the backslash/forward-slash that is needed by windows.

Here is an example of what box_path() is returning:
C:\\Users\\mbarajas\\Documents/Box/RES_Data/OISST/oisst_mainstays/

Here is what a correct path would be:
C:\Users\mbarajas\Box\RES_Data\OISST\oisst_mainstays