Remove SIMP and ContinuationSIMP structs

[mohdibntarek]

There are currently 2 workflows when using TopOpt. One uses the SIMP and ContinuationSIMP struct and one uses Nonconvex directly building and optimising the model. I former is legacy API from many years ago that's now redundant given the latter workflow. It's best to get rid of these old structs and just rely on a loop for continuation SIMP.

[mohdibntarek]

This is fixed by #126.