Have a `use_cvode` boolean argument for functions that run the cell

[AurelienJaquier]

Following comment in #24 , we should have a better way of enabling / disabling cvode for functions that run a blucellulab cell, because stochastic ion channel models are not compatible with cvode.
I propose that we should add a cvode boolean argument to these functions.
Once this is resolved, we could have the download_memodel function of entitysdk check the ion channel models and return a variable saying if we can use cvode or not.