Plot_fit legend is gross for fits with multiple terms

[pgkirsch]

Example:

[bqpd]

most simply, you could drop the parens and use ** instead of ^

then you could drop the third decimal place?

also maybe use u1 instead of u_1?

and a space for multiplication, like 0.857 u1^-2.15 + 0.153 u1^0.585

[whoburg]

If we used GPkit objects more heavily throughout GPfit, we could just use GPkit's formatting.