Correct dysfunctions & compatibility  interpolate / no interpolate logics

- 'next' & 'prev' buttons do not work whith slices  higher than file
time step
- Correct wrong results when interpolate factors ( slices per
update/slices per hours ) are not multiple or submultiple of hours
- Ensure accuracy when no interpolate situation (FS#1087)
- Correct dysfunction with Linux ( Combobox do not react as in Windows)
- Simplify and rationalize the code