Introduce Short Term plasticity for synapses

[sidharthanup]

Add STP - Short Term Facilitation and Short Term Depression. Introduce modeling of the coupling between residual calcium and neurotransmitter availability, and also the resulting change in conductance (weights are multiplied by the fraction of released neurotransmitters rather than spikes).

Adapted from the work of Tsodyks et al. (https://www.researchgate.net/publication/5512288_SynaptiC_Theory_of_Working_Memory)

Fixes: https://github.com/BindsNET/bindsnet/issues/444 Signed-off-by: Sidharth Anupkrishnan [email protected]