clean copy of griddiag.py

and tell mcidasv.rbi to use it.

[1720]
Simplest proof of concept: keepMetadata decorator
[1720]
Don't need GridMath override if we do it this way
[1720]
try to not die if first arg is not a flatfield,
or if there aren't any args.
[1720]
Clean copy of grid.py
And corresponding change to mcidasv.rbi
[1720]
Also works for e.g., makeVector formula
[1720]
Merge pull request #93 from mhiley/inq1720-decorator

implement inq1720 using decorator