handled optional dependencies in pyproject.toml as mentioned in #117

[neerja-1984]

the plotting dependencies have been made optional in the pyproject.toml file
the change made is

optional-dependencies.plotting = [ "cartopy", "matplotlib", "pandas-flavor" ]

additionally , the pandas-flavor library has been removed from the requirements.txt This ensures our plotting libraries are consistent throughout the code.

[ocefpaf]

@neerja-1984 please always add a description of your PR. You can edit the message above to create one. It shows that you put some thought into what you are doing and help future us to understand what are the changes here.