Missing data on maps due to coordinate difference (0-360 and -180-180)

[geek-yang]

This issue is spotted in PR #50 . When plotting maps using the data from cpm/rcm/gcm runs (from Petter Lind), some parts are missing. This could originate from the processing of data (calculating difference between future runs and past runs / interpolation / clipping) when the coordinate systems are different (0-360 and -180-180), which still needs to be further investigated.

[geek-yang]

One example about missing data for certain regions. It is quite likely that the original data does not cover the full region of target. It is better to mask the missing data area.

[geek-yang]

Another example about missing data around 0 deg, which is quite likely due to the different coordinate systems (0-360 and -180-180).