Add testing for 'cores' argument in sens_apsimx

[femiguez]

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

[femiguez]

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

[femiguez]

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)

[femiguez]

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

[femiguez]

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

[femiguez]

This seems to be working