Add cache management for _schwarz_screener_cache (unbounded memory growth)

[San1357]

Problem

_schwarz_screener_cache in gbasis/integrals/electron_repulsion.py is a
global dictionary with no eviction policy. During long sessions with different
basis sets and thresholds, it can keep accumulating entries leading to
unbounded memory growth.

Proposed Fix

One potential approach could be using weakref.WeakValueDictionary so cache
entries are automatically evicted when basis set objects are garbage collected.
Other options like LRU cache or a manual clear_cache() function could also
be considered — open to suggestions.

Related

• Raised in PR PR[9] (combine PR-1-PR-8) feat: combine weeks 1-9 implementations into single branch #245 review by @marco-2023
• Part of WoC: 2-electron integrals: better performance and more flexibility #221 (WoC: 2-electron integrals)