Support specifying general contraction schemes in the basis set

[awvwgk]

Is your feature request related to a problem? Please describe.

The basis set format allows the description of segmented basis sets, however for general contraction schemes the schema is suboptimal and would not allow programs with optimized code paths for generalized contraction schemes make use of them easily.

Describe the solution you'd like

A way to specify a set of contraction coefficients angular momenta for primitive exponents to specify general contraction schemes.

Describe alternatives you've considered

Implement custom detection logic to reduce a segmented contraction scheme to a general one, however this requires some heuristics to determine whether the segmented code path or general one might be more effective.

Additional context