patch rainydays divide by 0 error in micro_global()

[goldingn]

proposed patch to issue #22

I haven't copied this over to micro_global_grid() or micro_terra() where the code is duplicated.

Please also review to make sure it doesn't break the intended use of these functions (I just zeroed out rainfall if no raininy days, but might that mess up totals in some situations?)

[mrke]

thanks Nick - I fixed it a slightly different way by just checking for NA at the end of the construction of RAINFALL and setting any to zero so (and did this for micro_terra too).