[Feature Request] Fix deposition rate specified in time units

[tomasfbouvier]

Summary

The fix deposition command currently only allows to specify the rate at which atoms are inserted in timesteps. Is it possible to add an alternative parameter to specify the rate as a time delay instead?

Detailed Description

The fix deposition command insterts atoms every M timesteps. When using a fixed timestep one can control the time delay between insertions. However when using adaptative timestep the number of timesteps required for a time interval varies and therefore one can only approximately control the rate of deposition. Many depositions involve high energy particles for which adt is needed. In order to gain full control one needs to combine the previous commands with halting and for loops. It would be much simpler to have a parameter to control the time delay between depositions directly within the fix command.