Added support to allow eam history tapes up to 15 for maint-2.0

[wlin7]

This is to mirror PR #4892 for master to increase eam history tapes
up to 15 to accomodate more extensive output configurations.

[BFB]

[wlin7]

Hi @tangq , @beharrop , this is the same as #4892 but using a branch that is based on maint-2.0 and for maint-2.0. Please help take a quick look.

The changes should be the same as in #4892 -- note that the differences between master and maint-2.0 in namelist_definition.xml do not showed up in the review page. The other two files, cam_history.F90 and cam_history_support.F90 remain identical between master and maint-2.0.

[tangq]

@wlin7 , how did you make this PR without showing numerous older commits?

[beharrop]

Looks good to me. Thanks

[wlin7]

@tangq , because master and maint-2.0 have diverged, I used a new branch based on current maint-2.0 to have a clean PR for these changes.

[tangq]

I noticed that the "squash and merge" button is not grey for my account. Does this mean users who are not integrators can merge changes into the maint-2.0 branch?

[rljacob]

That should be fixed. Thanks for pointing it out.

[wlin7]

Merged to maint-2.0.