Definition of modifications and alterations

The case of schemes
Definition of dominant morphisms for algebraic spaces

This is just one possible definition
Definition of birational morphisms for spaces

This is just one possible way of extending the defition for schemes to
the case of algebraic spaces.
Change in terminology
Integral algebraic spaces

The definition as given in this commit is at least OK.
Modifications and alterations for algebraic spaces

Uses the definition of integral algebraic spaces from the previous
commit.
Starting to prove things about blowups of surfaces