Implement Timeseries in Fortran

The problem

To close #35 (closed), we need a Timeseries object to hold the ERF

Definition of "done"

• We have a Timeseries object which can be used as part of solving #35 (closed)

Additional context

See https://gitlab.com/magicc/fgen/-/blob/solver/tests/test-data/two_layer_model/two_layer_model_solver.f90?ref_type=heads#L97 for how we expect to use it. See https://gitlab.com/magicc/fgen/-/blob/solver/src/fgen_runtime/timeseries.py?ref_type=heads#L12 for sketch of how the API could look.