Add cache for locations and parameters

[d2hydro]

create file equivalent for the Locations and Parameters cache properties:

• method for reading/writing cache from files
• including a cache-outdated parameter to check if cache is to be refreshed
• include a periodic callback in bokeh.main to update cache (when outdated)

issue as solution for #49

[d2hydro]

Solves issues: #61 and #63

[d2hydro]

fixed in 0.6.0