Add Flowpipe type

[schillic]

We currently have AbstractReachSets for a single (convex) set and ReachSolution for a vector of those. The output of continuous-post operators is also a vector of AbstractReachSets. In hybrid reachability algorithms these vectors are mixed, which causes problems with vector types and also complicates finding individual flowpipes later (e.g., #261).

Proposal: Add an intermediate type Flowpipe. It works more or less like a ReachSolution but does not contain options. The ReachSolution would then become a vector of Flowpipes instead.