Previously, the `MapFieldMergeAverage` and `MapFieldPermutation` helper
function had to iterate over every possible type and create a separate
code path. This change uses the new extract component functionality to
create separate code paths only for different component types. This both
requires less code (the common filter library dropped from 66MB to 42MB
on my Mac) and covers more cases (such as `Vec`s larger than 4
components).
To make the implementation easier, `UnknownArrayHandle` now can create a
new `UnknownArrayHandle` of the same `ValueType` but with the basic
storage (so you can work with read-only storage) and the ability to
allocate the unknown array.
This function is compiled into the vtkm_filter library, so all filters
can implement this type of mapping without providing their own version
of compiling it.
Because we only compile it once, we provide more types to convert.
Hopefully the total compile time balances out.
