Bridge Closures

[martintb]

Many closures are expressed in terms of bridge functions. pyPRISM would benefit from having a system to easily handle closures in this form.

I propose having a BridgeClosure class and a Bridge parent class. The user would just create a BridgeClosure class and pass the selected Bridge class or a Bridge class object to BridgeClosure.init().

See the reference below for a coherent description of bridge closures.

The Journal of Chemical Physics 116, 8517 (2002)
https://doi.org/10.1063/1.1467894