Add testing for 'cores' argument in sens_apsimx

[Fernando Miguez]

Need to test this as some simulations seem to fail when this is used

[Fernando Miguez]

At the moment, I do not have a solution for perfect use of the cores argument. Some failures and different results might occur. Only available for unix, but will try to add an option for Windows. Another option would be to add back-end, using 'futures' for example

[Fernando Miguez]

Tasks:
- test cores argument in laptop with more than 4 cores
- develop Windows option
- add 'futures' as a back-end
- Track down source of error(s)

[Fernando Miguez]

Update: ran a sensitivity analysis with 78125 combinations in Linux (Xubuntu) without any failures using 6 cores (out of 8 available)

[Fernando Miguez]

Have I solved the issues I was having before? Need to test on other set ups and still need to develop Windows-compatible option

[Fernando Miguez]

This seems to be working