Use arbitrary length integers to timestamp snapshots

[pbartholomew08]

This will give snapshot files of the form

ux1 ux2 ux3 ... uxN

without zero padding, currently checkpoints/statistics have not been considered.

[pbartholomew08]

Replaces write(str, fmt) ... with str = ... where str is an allocatable character array - it isn't clear that the former is either correct or portable. (See https://github.com/xcompact3d/Incompact3d/pull/94#discussion_r795452667)

[slaizet]

Paul, is this formatting compatible with Paraview?

[pbartholomew08]

Some code in LES models had drifted out of consistency, merged current master into branch and tested with ParaView (20 snapshots covering single and double digits)