add British trad grades

[musoke]

British trad grades can have two parts: technical and adjectival. https://en.wikipedia.org/wiki/Grade_(climbing)#British

todo:

• British technical grades #170
• add British adjectival grades #171
• hybrid grades #169

[musoke]

This is tricky for two reasons:

1. Grades have both adjectival and technical components. This can be handled by adding a column for each and assigning the score of the combined grade to something like the intersection of score for each scale.
2. According to my understanding, the adjectival grades overlap. For example, according to both rockfax's table, E1, E2 & E3 are all plausibly equivalent to 5.10b. Table based conversions can't handle this.