Add .mol2 writer

[chrisjonesBSU]

We currently have a mol2 reader, but not a writer. We should include a writer as well for consistency with the other formats in gmso.formats. This will also allow us to use GMSO as the backend for writing mol2 files from mBuild.

[CalCraven]

Other readers that we might want to consider are: GROMACS top files, HOOMD GSD files.

[CalCraven]

Link for guidelines to the mol2 format:L
https://www.structbio.vanderbilt.edu/archives/amber-archive/2007/att-1568/01-mol2_2pg_113.pdf

[chrisjonesBSU]

That link seems to be broken. Maybe this would work instead

https://chemicbook.com/2021/02/20/mol2-file-format-explained-for-beginners-part-2.html

[CalCraven]

https://zhanggroup.org/DockRMSD/mol2.pdf This one is decent too. Unfortunately these are not necessary stable links in general since they're hosted by individuals. Thanks for noting this @chrisjonesBSU