Rewrite markers placement

[jgieseler]

Addresses #106. This pull request makes several improvements to the way plot markers and legends are handled in the solarmach plotting functions. The main focus is on enhancing the clarity and maintainability of marker annotations (especially numbered markers) and their representation in the legend. A new custom legend handler is introduced to support this, and redundant or complex code for manual legend positioning is removed.

Legend and marker handling improvements:

• Added a custom legend handler class _NumberedHandlerLine2D to display marker numbers directly in the legend, making numbered markers clearer and more consistent between the plot and legend.
• Updated the logic in both plot and plot_pfss methods to use this new handler for numbered markers in the legend, replacing manual annotation logic and simplifying code for marker handling. [1] [2]
• Refactored legend creation to dynamically build the handler_map and associate handles with the appropriate custom handler, improving maintainability and flexibility. [1] [2]

Marker annotation visual tweaks:

• Refactor the appearance of number/letter marker annotations on the plots by adjusting text offset and font weight, making the code future-proof by replacing to-be-deprecated matplotlib code. [1] [2]

Imports:

• Updated imports to include HandlerLine2D from matplotlib.legend_handler for use with the new custom legend handler.