v0.5.1

API changes

Unify API for constant sized objects and sizing of investment. For both, Flow and
GenericStorage, the argument investment is now deprecated. Instead,
nominal_value and nominal_storage_capacity accept an Investment object.
Change investment for experimental :class:oemof.solph.components.experimental._sink_dsm.SinkDSM: Remove
obsolete parameters flex_share_down and flex_share_up.

Add option to run multi-period (dynamic) investment models with oemof.solph as an experimental feature:
- You can change from standard model to multi-period model by defining the newly introduced periods
  attribute of your energy system. Be aware that it is experimental as of now. periods is a dictionary
  mapping the periods you want to model (usually years) to pandas.date_range objects.
- Add attributes periods to :class:oemof.solph._energy_system.EnergySystem.
- Introduce new Pyomo Sets PERIODS and TIMEINDEX in :class:oemof.solph.models.Model.
- Index all investment-related variables with PERIODS and flow variable with TIMEINDEX, which
  is a tuple of periods and timesteps.
- Add lifetime tracking for investment options by introducing the attributes lifetime and age.
- Add new investment-related variables total holding the total capacity, old holding capacity
  to be decommissioned, old_exo (for exogenous) holding existing capacity to be decommissioned and
  old_end holding model-endogenously installed capacity to be decommissioned after its lifetime.
- Include discounting and calculating annuities in the objective function terms. Introduce attribute discount_rate
  of :class:oemof.solph.models.Model and interest_rate for individual investment objects (options.Investment).
Add storage_level_constraint that allows to set flows from/to storage (in)active based on storage content.

See extensive documentation in user guide and API reference for the new (experimental) multi-period feature.