[Matlab/Examples] Make constant pressure reactor example more explicit

Both the reactor and the adjacent reservoir need to have the same pressure. This
example relied on the fact that the default pressure of the 'air' phase is 1
atm, which caused it to break if the user changed the pressure of the reactor
gas.

samples/matlab/reactor1.m

@@ -20,14 +20,16 @@ end
 
 nsp = nSpecies(gas);
 
+P = oneatm
 % set the initial conditions
-set(gas,'T',1001.0,'P',oneatm,'X','H2:2,O2:1,N2:4');
+set(gas,'T',1001.0,'P',P,'X','H2:2,O2:1,N2:4');
 
 % create a reactor, and insert the gas
 r = Reactor(gas);
 
 % create a reservoir to represent the environment
 a = IdealGasMix('air.cti');
+set(a,'P',P)
 env = Reservoir(a);
 
 % Define a wall between the reactor and the environment and