ops: kokkos: revise `extent` overload constraints

refs #522

Kokkos extent overloads:

function	parameter types
`extent(const T & objectIn, const IndexType i)`	Kokkos view or expression

Unit tests (in tests/functional_small/ops/):

file name	test name	tested type
`ops_kokkos_vector.cc`	`ops_kokkos.vector_extent`	`Kokkos::View<double*>`
`ops_kokkos_diag.cc`	`ops_kokkos.diag_extent`	`pressio::diag( Kokkos::View<double**> )`
`ops_kokkos_span.cc`	`ops_kokkos.span_extent`	`pressio::span( Kokkos::View<double*> )`
`ops_kokkos_matrix.cc`	`ops_kokkos.dense_matrix_extent`	`Kokkos::View<double**>`
`ops_kokkos_subspan.cc`	`ops_kokkos.subspan_extent`	`pressio::subspan( Kokkos::View<double**> )`

Pressio / pressio