Make run optional in execute

[brey]

pyPoseidon/pyposeidon/schism.py

Line 748 in d1ee4fa

self.run(**kwargs)

Use a flag in execute to update/make model without executing.

[pmav99]

somebody could argue that calling .execute() without executing implies that we need to rethink the names of the methods.

[brey]

Agree. We need an additional build() function.