✨ Add new Enum type: RestartType

[ymzhang0]

Here I introduce a new enumeration type RestartType to control the starting point of epw.x calculation. Now it supports three kinds of checkpoints:

RestartType.wannierize: start from scratch, it will set elph=true and ep_coupling=true and epbwrite, epwwrite=true. So the transformation from coarse Bloch basis to Wannier basis will be executed.

RestartType.ephwrite: It will do interpolation from Wannier basis to fine-grid Bloch basis and write the matrix element into the prefix.ephmat folder.

RestartType.ephread it will assume that the interpolation is finished and prefix.ephmat folder is properly written.

Currently RestartType is well defined for Eliashberg calculation. For transport calculation it might need to be modified.