Make astype set new nodata by default, and data.setter check for unmasked nodata

[rhugonnet]

That little change on astype ended up being a lot bigger than I anticipated!

I realized that:

• New nodata value could be created at any point without being masked, see Creation or numerical operation can create unmasked nodata value in Raster without notice #469,
• Some arithmetic overloading casted nodatas to None, see Arithmetic overloading replaces existing nodate with None for certain operation with int data types #471,
• We still had the masked option for Raster which is deprecated, see Remove masked option in Raster.__init__ as it is no longer supported? #470.

What's changed

1. At the end of data.setter, we now check that there isn't any nodata value that is unmasked, otherwise we mask them (in case it is user input and not numerics) BUT RAISE A WARNING and recommend the user to either: Mask his array when creating the raster (if that was what he was doing), or Change the dtype to not have numerical operation derive the nodata value (if that was what he was doing). This mostly happens with int, and should be very easy to silence using a float array with properly masked values (or NaNs).
2. Changes the nodata when using astype by default. If the nodata is already in the array, it will be detected by set_nodata().

Surprisingly, those two "big" changes didn't create so many new errors in tests, and actually rather allowed to identify a couple bugs! 😄
The rest is adding new tests, silencing or catching warnings, and updating the warning for the nodata setter (that were still not so clear when I re-read them).

Resolves #471
Resolves #470
Resolves #469
Resolves #394

[adehecq]

Good job finding all these bugs !
I went quickly through the tests, but the changes make sense.