PINNx vs DeepXDE

[lululxvi]

Thank you for sharing the library. I find that PINNx uses a lot of code from DeepXDE. I am OK with this, but it should be clearly acknowledged in the source code and README, in addition to "inspired from [DeepXDE](https://github.com/lululxvi/deepxde".

Also, you and PINNx team are very welcome to contribute code back to DeepXDE. I am happy to merge the new code/design from PINNx to DeepXDE, and maintain the code together.

[chaoming0625]

Dear Lu Lu, thanks you so much for your message. I'm sorry for not prominently thanking DeepXDE, but in fact, we clearly stated in About PINNx that pinxx is rewritten according to DeepXDE. I will add this line in README.

We are also highly interesting in merging pinnx into DeepXDE. However, due to architecture and recent events, this matter has been delayed. My initial idea is to add a new submodule deepxde.pinnx, and reuse some of the existing deepxde functions.

I will pull a PR and try to integrate pinxx into deepxde in these days. I am looking forward to your response to this upcoming PR.

Thank you once again.

[chaoming0625]

I add acknowledgements in most .py files in pinnx to acknowledge the code is rewriten from DeepXDE. See #20

[chaoming0625]

Dear Lu Lu, please see the PR lululxvi/deepxde#1932 for details, in which I drafted a version of merging.

[lululxvi]

Sounds great.