Allow passing TINs to GEOS functions

[komzpa]

GEOS functions now fail with TINs.

They could work if we represent them as GEOMETRYCOLLECTION(POLYGON(...)...).

[komzpa]

In 17396:

Allow passing TINs to GEOS functions.

Closes #4378
Closes ​https://github.com/postgis/postgis/pull/392

[mdavis]

Ideally the docs should be updated to reflect this change.

[dbaston]

It might be more clear have a function to convert a TIN to a GeometryCollection on the PostGIS side, rather than silently changing the type when passing it to GEOS.

[mdavis]

Perhaps having both is good?

An explicit PostGIS function is good for explanatory purposes (and might support other functionality in the future). It also might be useful for converting the TIN type to a simpler WKT type for output purposes or compatibility with other systems.

But surely there's no harm in auto-converting as well?