Make every header self-contained

[susilehtola]

None of the library headers could be relied upon to compile on their own;
they worked only when something else had already pulled in the standard
headers they use. Compiling each non-data header in isolation produced
eight hard failures:

• spherical_micro_batcher.hpp calls std::any_of, std::minmax_element,
  std::partition, std::sort, std::unique and std::set_union without
  .
• product_quadrature.hpp declares std::unique_ptr members without
  , so the class had no members at all when compiled alone. This
  cascaded into spherical_quadrature.hpp, pruned_spherical_quadrature.hpp
  and the batcher.
• util/create_array.hpp uses std::array and std::index_sequence,
  util/copy_grid.hpp uses std::copy, util/legendre.hpp uses size_t, and
  util/bound_transform.hpp uses assert and std::tuple, none of them
  declared.

Separately, was not included anywhere in the library, although
std::cos, std::sin, std::sqrt, std::pow, std::log and std::abs are used
throughout and M_PI appears in nine places. The two Newton solvers also
use std::numeric_limits and std::runtime_error without or
.

Add the includes each header actually needs. Also qualify the one
unqualified cos() call in gausslobatto.hpp: is not required to
place names in the global namespace, so it only resolved because <math.h>
happened to be reachable.

Every non-data header now compiles standalone.

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01FDTFYJMQ76iujDFNHzZyXF