Make PointInCell regridding scheme public

[pp-mo]

That is, currently iris.experimental.regrid.PointInCell.
We anticipate this being used widely for ocean data regridding.
We should make this public so people can have confidence in using it, even for production processes.

Supporting factors:

• it is worth promoting, as it solves some major problems being one of the few schemes that works with 2-D X and Y coordinates
• we are now advising people to use this, so we would not contemplate actually removing this facility
• while the underlying code can remain experimental, or private, the scheme class itself has a very straightforward API that is unlikely to change : http://scitools.org.uk/iris/docs/latest/iris/iris/experimental/regrid.html#iris.experimental.regrid.PointInCell

[pelson]

@pp-mo - I've removed the v2.0.0 milestone from this issue. I'm entirely in favour of this, but would like to have a clear milestone of what has to arrive in v2.0.0. This change does not meet that criteria. Is there some other milestone you'd like to log this under, or a label/some other means of remembering to get around to doing this?

[bjlittle]

@alastair-gemmell See here for examples of schemes that are public and part of the core of Iris.

As a courtesy, also see iris.experimental.concatenate for an example of the exception to raise to users when they attempt to use functionality that has migrated from experimental to the core of Iris.

[lbdreyer]

Closed by #3390