Remove base_plt

[RemDelaporteMathurin]

https://github.com/fusion-energy/regular_mesh_plotter/blob/c5dbff275d56aaef8ec65fd7d85fc43091e6e8ee/regular_mesh_plotter/core.py#L31-L36

We don't need this as this is redundant and removes flexibility

[shimwell]

I think this allows one to combine two plots together, I believe the dagmc-slice-plotter makes use of this.

[RemDelaporteMathurin]

this (combining plots together) is natively done in matplotlib, not need to have these lines

[shimwell]

I also recall some issue with plotting two separate plots in one script. They use to be combined unintentionally.

But yes feel free to remove/ refactor

[shimwell]

done