Rename BRiM to SymBRiM

[tjstienstra]

This PR renames BRiM to SymBRiM as discussed in #140.

The current name of the package is BRiM: Bicycle Rider Models. The new name will be SymBRiM: Symbolic Bicycle Rider Models.

The main reasons to rename BRiM to SymBRiM are:

• The word BRiM is also used in different contexts as a word and an abbreviation, both of which are impractical occasionally.
• SymBRiM is a (more) unique name.
• The name also seems to make releasing it more difficult, refer to #37.
• SymBRiM is more complete, as it also covers the fact that we are focusing on deriving the symbolic equations of motion.
• SymBRiM has two syllables, which is also giving a practical alias sb. As far as I know, there is no other package using this alias.

There are also a few disadvantages of renaming:

• The package has been published under the name BRiM in the scientific community, i.e. the BRiM paper
• Though redirects can be set up (mostly automatically if done right), they can be broken.
• Takes some effort, but I'm willing to do it.

[tjstienstra]

Reason that the doctests fail is because they have to install the symbrim tutorial environment, which expects this repository to be under the name mechmotum/symbrim.