Mask and/or dots for metrics visualization (new MR)

Original MR description (!140 (merged) accidentally merged, dev-vis-mask-2 was created from dev-vis-mask):

Hi @vagudets

I want to obtain two types of skill metric maps in each execution: one with dots and another making non-significant grid points. I did this development in my working branch, but it may be useful to include it in SUNSET for other users.

Visualization() accepts the significance parameter as logic or string (i.e. mask, dots or both). The mask option is only available in Robinson plots, so keeping T/F is still useful. For the other cases, the main difference is the file name.

I have included some tests in one of the scripts to show how these changes affect.

Kind regards,

Núria

FYI @nperez