[newchem-cpp] Introduce GRIMPL_NS macro

[mabruzzo]

to be reviewed after #471 is merged

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A lightweight PR that introduces the GRIMPL_NS and GRIMPL_NAMESPACE_DECL macros.

Description

The basic idea is that we will gradually replace

• every occurrence of grackle::impl that is used to open a namespace block with GRIMPL_NAMESPACE_DECL. After the replacement, a namespace block looks like:

  namespace GRIMPL_NAMESPACE_DECL {
  // ...
  }  // namespace GRIMPL_NAMESPACE_DECL

• All other occurrences of grackle::impl will be replaced with GRIMPL_NS

As a part of this PR I demonstrated what this will look like in the ceiling_species.hpp file.

The impetus for doing this sooner rather than later is to start using the macros in newly created C++ files.

Motivation

These macros are inspired by similar macros used to define LLVM's implementation of the standard C library.

I provided really detailed docstrings explaining why we want this but it essentially boils down to

1. widespread use of GRIMPL_NS lets us rename grackle::impl namespace. This affords a few benefits (with the benefit of hindsight, I slightly regret grackle::impl as a name, and it's probably important for distributing Grackle as Header-Only library). It's also goes hand-in-hand with the GRIMPL_NAMESPACE_DECL macro
2. the GRIMPL_NAMESPACE_DECL macro is a relatively elegant way to control symbol visibility in shared libraries

Other thoughts

I generally think that introducing GRIMPL_NS is a good idea. But, if you aren't convinced, I could remake the PR without GRIMPL_NAMESPACE_DECL.