Make the batcher and partition iterators usable

[susilehtola]

SphericalMicroBatcher::const_iterator declared its comparison operators
taking the non-const iterator type:

bool operator==( iterator other ){ ... }

There is no conversion between the two, so comparing two const_iterators
was a hard error and the loop the class exists to support could not be
written over a const batcher. A const batcher also had no begin()/end()
at all, only cbegin()/cend(), so range-for over one failed by a second
independent route.

RadialGridPartition::rgp_iterator had the same class of bug in its
post-increment, which declares iterator retval = *this; -- the enclosing
class's non-const typedef, so const_iterator::operator++(int) could never
instantiate.

Fix both, and give the batcher const begin()/end().

While here, three related defects in the same iterators:

• operator+ mutated *this and returned a reference to it. That is
  operator+= under the wrong name; at() worked only because it discards
  the iterator afterwards. Return a copy and take a difference_type.
• difference_type was spelled different_type and iterator_category was
  spelled iterator_catagory, so std::iterator_traits saw neither and no
  standard algorithm could use these iterators.
• With the traits now visible, reference = value_type& would have been
  a trap: operator* returns a prvalue, so binding a reference to it
  dangles. Use the proxy-iterator spelling instead (reference =
  value_type, pointer = void).

range() and operator* are now const, as they never mutated anything.

The existing test called cbatcher.at(i) but never iterated a const
batcher; add both a range-for and an explicit cbegin()/cend() loop.

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