make restarts using CHGCAR/WAVECAR easier

[zhubonan]

workflow engine should workout the needed type of restart and set the INCAR tags accordingly. This may introduce another input settings for VaspWorkChain though.

Choose from:

• restart with WAVECAR for wavefunction
• restart with CHGCAR for the charge density without WAVECAR
• no action - leave the INCAR tags as they are

The default should be "use as much as possible". The content of the remote folder can be obtained during the workchain operation to check which files are present for restarts before setting the parameters.