Changes related to "flameSpeed" and "dfChemistryModel"

[xu-23]

The past flameSpeed is only used when the time step is 0.001s. Now, we give two choice. If you choose flameSpeed, the time step will be set as the same as it in controlDict. If you choose flameSpeed1, you can input a float number as the time step.

In the current version, when solving Yeqn with DNN methods, all the points will be chosen as the problem size, and we then solve all points whose temperature are above frozenTemperature. This process makes Chemistry time ratio over 60%. If we read frozenTemperature before we decide problem size, Chemistry time ratio will be 10%.

[zhixchen]

You should now be able to add reviewers for your PR.