Add easy import surface for std-compiling + SpecResults, and expose runtime __version__

[Yong2Sheng]

Summary

I improved ergonomics across the repo by defining a clearer “public” import surface and making version introspection available at runtime.

This PR includes:

• An easy-import layer for standard star compiling
• The same easy-import pattern for SpecResults in plot_spec_file.py
• A lightweight __version__ attribute backed by distribution metadata

What changed

1) Easy imports for standard star compiling

• I added a small re-export layer (via __init__.py) so the most commonly used standard-star compiling entry points can be imported from a higher-level namespace.
• This reduces deep import paths and makes notebooks/scripts cleaner.

2) Easy imports for SpecResults

• I applied the same pattern to SpecResults in plot_spec_file.py so I can import it without reaching into internal module paths.
• The intent is to make SpecResults feel like a first-class public API object.

3) Runtime __version__ from packaging metadata

• I exposed __version__ using importlib.metadata.version("researchcodes"), so the version comes from the installed distribution metadata rather than being duplicated in code.
• This keeps version as a single source of truth (defined by packaging metadata) while still allowing researchcodes.__version__ at runtime.

Why

• As the repo grows, deep import paths become harder to remember and more fragile during refactors.
• A stable, intentional import surface improves usability and makes future reorganizations less disruptive.
• Surfacing __version__ via metadata avoids maintaining duplicate version strings and aligns with modern pyproject.toml-based packaging.
• But some less used functions for specific tasks such as paper revisions are still kept in the notebook or a module script in the same folder where the notebook lives.

How to test

# Standard star compiling (examples)
from researchcodes import write_std_h5  # plus other top-level re-exports, if applicable

# SpecResults (examples)
from researchcodes import SpecResults  # if re-exported at the package top level
# or
from researchcodes.plot_spec_file import SpecResults  # if re-exported at a module-level __init__.py