Use ExponentialUtilities Propagator

[goerz]

Allow method=ExponentialUtilities as a piecewise propagator that delegates to ExponentialUtilities.

This is blocked by SciML/ExponentialUtilities.jl#176

[krugx]

This would be incredible. I've been trying to get GRAPE.jl to work with a ExponentialUtilities.jl based propagator but had a lot of trouble with with convergence Reason for termination: ABNORMAL_TERMINATION_IN_LNSRCH.

[goerz]

Closed by #97