[Cython] Add declarations for free functions to _cantera.pxd
[Equil] Remove a calculation with an unused result
[Equil] Avoid floating point overflow from pressure term

Since computing the residual norm effectively requires p**2, the
correct limit to avoid an overflow is sqrt(DBL_MAX), or roughly
exp(300).

This change fixes crashes due to SIGFPE in cases where floating point
exceptions have been enabled, and also fixes a few cases where the
solver did not previously converge.
[ck2cti] Show line number for some parsing errors