fgrad with pytrees

[ASEM000]

This PR adds the ability to differentiate arbitrary pytrees using fgrad, value_and_fgrad

import finitediffx as fdx

params = {"a":1., "b":2., "c":3.}

@fdx.fgrad
def func(params):
    return params["a"]**2+params["b"]

func(params)
# {'a': Array(1.9995117, dtype=float32),
#  'b': Array(0.9995117, dtype=float32),
#  'c': Array(0., dtype=float32)}

[codecov]

Codecov Report

Patch coverage: 94.36% and project coverage change: -0.43 ⚠️

Comparison is base (2ce5d2a) 98.82% compared to head (35f731d) 98.40%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #4      +/-   ##
==========================================
- Coverage   98.82%   98.40%   -0.43%     
==========================================
  Files           6        6              
  Lines         594      625      +31     
==========================================
+ Hits          587      615      +28     
- Misses          7       10       +3     

Impacted Files	Coverage Δ
finitediffx/_src/fgrad.py	93.52% <93.22%> (-1.31%)	⬇️
tests/test_fgrad.py	100.00% <100.00%> (ø)

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