convert from fmpq_mat to Matrix{Rational}

[cossio]

[wbhart]

Thanks again! In both these pull requests, it makes sense to write a test which converts to a Julia matrix and then back again and checks if the result is the same.

You would add a new test function for this in Nemo/test/flint/fmpq_mat-tests.jl, etc.

This way, Travis-CI would be testing something nontrivial.

Also, you might want to add a section on conversions in doc/src/matrix.jl You'd need a docstring on the actual functions. (We can run the documentation parser once the docs exist.)

[cossio]

Thanks. I will take a look at the tests and the docs tomorrow.