add conversion for list type
PDL `ndims = 0` when vector length is 1

When the length of a (real or integer) vector is 1, return a PDL with
ndims = 0 so that it is scalar PDL rather than a PDL vector of length 1.
For example, the R values `42` and `c(42)` both get mapped to the PDL
`long(42)`.
test for perl data from lists should be list with PDLs inside
compare nested structures using `sdump()`

We use `PDL::IO::Dumper::sdump()` to compare nested structures
containing PDLs because of how multidimensional PDLs in conditionals
interact with `Test::More::is_deeply()`.