Improve trends

[brownsarahm]

Adds parameters to each trend:

• solves backend implementation of #145 by adding detail_view_type parameter
• adds overview_legend parameter to enable vizualisation improvement and using separate scales based on trend type
• adds display_name parameter to improve display quality of #153

Also renames roles to clarify user confusion: trend - > (independent, dependent) groupby - > splitby

and renamed resutl_df headings to match: feat1-> independent feat2 - > dependent

required additionally changing logic around symmetry of regression and correlation trend types