added higher level conversion function to exec method

What new features does this PR implement? Please summarize the features that this PR implements. As relevant, please indicate the issues that the PR closes.

Creates a new library entitled spatial which handles logic related to spatial simulations and spatial visualization preparation with the simulariumio package.
Implements a data model for the conversion of spatial simulation output data to .simularium format.
Implements the spatial library within the various combine exec methods such that they may be publicly ported.

biosimulators / Biosimulators_utils

added higher level conversion function to exec method #150