added direct support for Basis-Set-Exchange

[Davda-James]

This PR adds support for loading Gaussian basis sets directly from the Basis Set Exchange (BSE) via a new parse_bse parser in gbasis.parsers. The implementation converts BSE data into the same internal format used by existing parsers, enabling seamless integration with downstream APIs such as make_contractions.

Added tests

test_parse_bse_sto3g
test_parse_bse_empty_elements
test_parse_bse_unexpected_coeff_layout

Checklist

• Write a good description of what the PR does.
• Add tests for each unit of code added (e.g. function, class)

Type of Changes

| | Type |
| ✓ | ✨ New feature |

Closes #209