Defer loading element data until attribute access

[paulromano]

Here's my quick attempt to reduce the overall import time by deferring the autoload of element data until one actually accesses the corresponding attribute from the mendeleev module, e.g. mendeleev.Li. This seems to work pretty well in my limited testing. The only downside I can see is that the attribute names (H, He, etc.) won't show up in an autocomplete from an IDE. @lmmentel Let me know what you think of this approach.

Fixes #118

Import timing master branch (python3.12):

> time python -c "import mendeleev"
python -c "import mendeleev"  3,78s user 0,78s system 125% cpu 3,624 total
python -c "import mendeleev"  3,67s user 0,85s system 125% cpu 3,588 total
python -c "import mendeleev"  3,86s user 0,85s system 124% cpu 3,782 total

Import timing current branch (python3.12):

> time python -c "import mendeleev"
python -c "import mendeleev"  0,70s user 0,75s system 278% cpu 0,522 total
python -c "import mendeleev"  0,64s user 0,80s system 280% cpu 0,516 total
python -c "import mendeleev"  0,68s user 0,78s system 278% cpu 0,524 total

On average this amounts to 7X speedup of import time.

[kalvdans]

I rather see a deprecation of these, and force people that want direct access to use mendeleev.elements.H instead of just mendeleev.H.

[lmmentel]

Nice idea with the lazy loading, does it only work with dot access or also when importing with from mendeleev import Li ?

If it's the latter I think I would rather allow for sth like:

from mendeleev.elements import H, O, Fe

or

from mendeleev.elements import *

Few more places worth updating:

• [x] update docs on data access
• [x] ideally we would also have an deprecation warning when someone tries from mendeleev import H

[paulromano]

The lazy loading works with either dot access or the from mendeleev import ... form. I'll go ahead and update this to require the import specifically from mendeleev.elements and update docs/tests accordingly.

[lmmentel]

I added a few adjustments, mostly catching up the latest changes on master and appeasing the linter.