Previously, you could create a FrameRange object with a negative step,
but it would go into an infinite loop if you tried to get the frame list

Given that we have the ReversedFrameList that achieves the same as a
negative step, it felt like using that was the correct approach.

Note that we still include the support for the negative step in the
parse method, so that we can provide a more useful error message during
init.

FrameRange : Prevent creation of FrameRanges with negative steps

`__dataTypesConversionDict` contains an item that returns a `None` value
which would cause an error.

When converting a list, it was simply using the first match, which would
often not be the desired one. Ex: converting `["abc", "b"]` to
`IECore.CharVectorData( [ 97, 98 ] )`, instead of
`IECore.StringVectorData( [ 'abc', 'b' ] )`.

The solution here is to use, on lists, the already existing flag on the
`__dataTypesConversionDict` entries that indicated the preferred
IECore.Data type for a given source.

The options selected here as the preferred one when there was ambiguity
were the same ones as returned by the simple (non-vector) types.

That required a small update to `dataTypeFromElementType()` in order to
avoid returning a random return, now that `list` entries in
`__dataTypesConversionDict` can have a preferred flag set to `True`.

…orLists

Data type conversion support for lists

Merge RB-10.4 to RB-10.5