v0.2.1

Release changes:

The new release adds the function merge_models to run simulations with complex network structures and multiple populations.
Examples are moved to the SNNExamples repository.

Other changes include:

• Added time::Time for tracking time across sim! and train!. plasticity! and record! methods call it in the arguments.
• Common plasticity rules defined for SpikingSynapse and CompartmentSynapse (<: AbstractSpikingSynapse).
• Neuronal activity is recorded in :fire with spike times. Functions to process firing rate are added in src/spikes.jl and src/populations.jl.