allow for multiple model blocks

Allow for more than one model block in a .mod file. The model equations would be concatenated by the preprocessor.

At the same time, allow for the replacement and removal of equations that have appeared in previous model blocks. We could say that if two equations appear with the same equation tag the second one takes precedence. As for removal, we would need to think of a syntax. Perhaps just an equation tag with no following equation would remove any previous instance of that equation tag. To be decided.

When this is done, we should formalize in the preprocessor the equation tags creating a table that associates a name with a the index of the equation it represents. This would make lookups easier in the preprocessor. This may also require us to create default equation tags to identify equations. To be decided.