Feat/fix morphs without modifying object

[sanjayankur31]

[sanjayankur31]

Gotta test this out a little more to confirm it's doing the right things.

[sanjayankur31]

@pgleeson this is ready now I think.

• it also tries to load referenced bits from included files (but not recursively---should it be looking recursively?)
• throws errors if it cannot find referenced bits
• adds a few tests for the function

It overwrites the NeuroMLDocument object by default, but perhaps it shouldn't in case users then continue to use it and end up writing it to file or something without realising that we've changed their model here?

[pgleeson]

Looks good to me!