API: pdf, cdf, logpdf multiple values support

The current implementation of pdf, cdf, and logpdf allows computation at only a single point. Add support for inputting multiple values.

Example: 
```python
mixture = NormalMeanVarianceMixtures(**mixture_params)
values = np.linspace(-1, 1, 40)
rqmc_params = {"error_tolerance": 0.001, "i_max": 300}
mixture.pdf(1,  rqmc_params) # One point
mixture.pdf(values, rqmc_params) # Multiply points
```