Clean up the LPdiag tool

[glatterf42]

As described by @khaeru here, there are some small improvements to the LPdiag tool that we can make and should not forget about:

• [ ] Copy-edit the docs page. There are several small enhancements that could be made, like using Sphinx roles instead of double-backtick in many places, and improving the clarity of text about the locations of files. In particular, the command no longer needs to be run in any particular directory as long as the user gives valid values for --wdir and --mps.
• [ ] Define, type, and comment the class attributes of LPdiag outside of the __init__() method so they appear in the docs; use that method only for setting defaults.

[macflo8]

I would like to add a minor thing that I discovered yesterday:

The default value of the --up-tail click_option passed to the lp-diag tool is set to -5. The help string says that it is set to 5. It seems that the minus slipped in there.

https://github.com/iiasa/message_ix/blob/2d7b3f538d7a99edb8539fdc8831e5230e2ee23d/message_ix/tools/lp_diag/cli.py#L50

[glatterf42]

Thanks for pointing this out!