wt_scl is sometimes a string, sometimes a float

[jdavies-st]

The interface for wt_scl is not clear. in dodrizzle is described as a float, but everywhere else it is a string. And sometimes it's an empty string. This interface needs to be cleaned up and made consistent.

Additionally, within the C code, it is wtscl, but the interface presents it as weight_scale.

[mcara]

Fixed via #134