[Feature Request] Enable assignment of printed system() calls to variable

[samuelhocking]

When using system() to pass shell commands, it is not currently possible to assign the printed output to a morpho variable.

E.g. var now = system("date +'%m/%d/%Y'")

[softmattertheory]

Really great suggestion, @samuelhocking. We can include it in the System class. I'll investigate how we can capture this.