Issue with R version 3.6.1 - 3.6.3

[henriquesposito]

Because this R version uses factor instead of character as default, this causes some functions, such as as_matrix(), to fail. There are two solutions for the issue. We can require in the description file of migraph that users have an r version that is 4.0 or above. Or, alternatively, we can make sure the functions work with both versions by adding if else statements to the functions.

[henriquesposito]

For the time being, I have updated the R version {migraph} depends upon on the description file to R vesrions 4.0 and above.