Add support for writing MRC files

[tluchko]

Following up on #100, I would like to write MRC files, as well as read them.

[orbeckst]

help (especially in the form of PRs!) is very welcome!

[Pradyumn-cloud]

I want to work on this issue. I tried to implement write feature of MRC file by adding write() method to the MRC class that saves grid data to MRC files. It properly reverses the coordinate transformations from reading, preserves the original file's header information when available, and integrates with Grid's export system so users can simply call g.export("file.mrc", file_format="mrc") to save their data. can i do a PR?

[orbeckst]

Thank you for contributing @Pradyumn-cloud ! Please submit a PR and link it to the issue. Include some tests and check that your patch is covered.

Once you have a PR up, tests pass, and test coverage shows that your code is covered by tests, please ping me @orbeckst with a request to review. I can't promise to review immediately as I am quite busy before the upcoming holidays but I'll try.