feat: add `diffpy.apps` to `diffpy.cmi`

[ycexiao]

Pack description

User applications to help with tasks using diffpy packages.
https://github.com/diffpy/diffpy.apps

Pack submission checklist

• Fork and clone this repository and cd to the top level by running cd diffpy.cmi.
• Create a requirements file by running touch requirements/packs/<your-pack-name>.txt. List all dependencies for your pack.
• (OPTIONAL) Create a directory to place your examples, if applicable, by running mkdir docs/examples/<your-pack-name>/<your-example-name>.
• (OPTIONAL) Copy your example scripts and data under the directory you just made. If you have multiple examples, house them under their own separate directory (i.e. .../<your-pack-name>/example1 and .../<your-pack-name>/example2).
• (OPTIONAL) Make a file under docs/source/tutorial/ call <your-pack-name>.rst with cp docs/source/tutorial/core.rst docs/source/tutorial/<your-pack-name>.rst. Add your examples to the file in the same format as listed.
• List your pack and its dependencies under docs/source/available-packs.rst using the same format as the other packs. In this same file, add a description of your pack (1-2 sentences is recommended).
• Give yourself credit by listing your name and contributors to the pack you've created!

Local testing

• Test to make sure the addition of your new pack worked by first creating a new environment and installing from source. Do this by running the following commands:

conda create -n testcmi-env diffpy.cmi
conda activate testcmi-env
pip install -e .
cmi install docs tests

• Run cmi info to see if your pack and any examples you may have added are listed there.
• Build and open the docs with the command cd docs && make html && open build/html/index.html. Navigate around the docs to make sure everything looks okay.
• Lastly, install your pack with cmi install <your-pack-name>.

When the above items are complete, make a pull request (PR) with the changes, referencing this issue in the PR (i.e. "closes #issue-number").

[ycexiao]

It is not ready until a newer version of diffpy.apps with dsl(domain specific langguage) support is released. This feature allows to launch a refinement task according to a .dpi-in file like below:

load structure ni from "Ni.cif"
load profile exp_ni from "Ni.gr"
set ni spacegroup as auto
set exp_ni q_range as 0.1 25
set exp_ni calculation_range as 1.5 50 0.01
create equation variables s0
set equation as "s0*ni"
save to "results.json"

variables:
---
- ni.a: 3.52
- s0: 0.4
- ni.Uiso_0: 0.005
- ni.delta2: 2
- qdamp: 0.04
- qbroad: 0.02
---