Add .mailmap file

[zyzzyxdonta]

Hi!

I felt like doing a bit of coding during the holidays. I'll probably do some more pull requests after this one (depending on how it goes 😄), so I thought I'd use this little cosmetic change as an announcement. 😁 Please don't feel pressured to look into any of this during the holidays.

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This pull request adds a .mailmap file which helps both tidy up the git log as well as improve the display of contributors on GitHub.

Here's the output of git shortlog -esn before adding the file:

   244  Matthias Werner <[email protected]>
    80  Peter Steinbach <[email protected]>
    76  Peter Steinbach <[email protected]>
    47  Pape, David (FWCC) - 139658 <[email protected]>
    36  Matthias Werner <[email protected]>
    14  David Pape <[email protected]>
    11  M. Werner <[email protected]>
     8  Mario Emmenlauer <[email protected]>
     7  tdd11235813 <[email protected]>
     6  Peter Steinbach <[email protected]>
     6  steinbac <[email protected]>
     5  David Pape <[email protected]>
     5  Steinbach, Dr. rer. nat. Peter (FWCC) - 120595 <[email protected]>
     4  David Pape <[email protected]>
     3  Matthias Werner <[email protected]>
     2  steinbac <[email protected]>

and afterwards:

   301  Matthias Werner <[email protected]>
   175  Peter Steinbach <[email protected]>
    70  David Pape <[email protected]>
     8  Mario Emmenlauer <[email protected]>