The complex stellar population subpackage is meant to combine bpass 
variables with a stelalr formation history to generate more complex 
populations. To this end, we're implementing a stellar formation history 
class. The basic SFH class for custom SFH is implemented using scipy 
splines. This basic class is also tested.

This method allows for the calculation of the event rate at a given lookback time. It uses the BPASS binning for this. 

This method is much faster, but suffers from a lack of accuracy.

Both functions are implemented for the BPASS spectra. 
However, the method is similar to the BPASS event rate calculation, which is sub optimal for the spectra calculation. It takes an extremely long time to do the calculation. A for loop of 100000 iterations is present, because each wavelength is seen as it's own "event type". It should be possible to remove this by implementing a function to take the separate wavelengths as a single unit. It would be interesting so see the influence of putting the for loop within a numba function, but thinking about restructuring the function is probably better. 

Including a "caching" of the BPASS spectra into a pickled DataFrame. 



Additional test files are required for this to run.

Instead of requiring a scipy interpolated spline as input. These functions now take a list of functions as input for the stellar formation and metallicity history. 

This adjustment has been made by using `numpy.interp` instead of scipy.interpolate. Furthermore, instead of scipy's spline integration `numpy.trapz` is now used to calculate the mass per bin.

…variable names

- Adds BPASS IMFS to the constants
- Adds check to BPASS IMFS
- Adds test for `data_compiler.SpectraData`
- Reduces data usage for rate tests

Uses `unittest.mock` to allow for a smaller DataFrame to be used.

The compiler now outputs the BPASS spectra as a 3D `numpy.ndarray` this minimises storage and allows for faster and less memory hungry operations than the MultiIndex pandas DataFrame. 

The tests are updated accordingly and using unittest.mock a single spectra file is use. (This could be simulated too)

The documentation now shows the size and shape of each input and output variable. 

The `_over_time` and `_at_time` for `CSPSpectra` and `CSPEventRate` are now able to take the `SFH` object as an input.

The CSP class in `utils` was importing the SFH class, which was importing the `utils` module. Therefore, failing importing. Thus, the CSP class has been moved to a separate file

at_time now uses 1000 as the default sample rate. 
The BPASS bins can still be used is a negative number is given. 
The `load_spectra` and `load_rates` have been moved to `hoki.load` and renamed to `all_spectra` and `all_rates`, respectively.
tests have been adjusted accordingly

Implements 2 new functions:
- A custom numba trapezoidal function
- A optimised mass_per_bin function for non-function-based data

The original event rate calculation took functions as in input. This becomes cumbersome when you have data at specific time values. This calculation is done by taking a 2D matrix with time points on one axis and BPASS metallicities on the other. The values of the matrix describes the SFR at that moment in time. 

The calculate_rate_over_time function has been adjusted to use this new function.

Small bugfix in `all_spectra`
The binary `kwarg` was not passed on to `SpectraCompiler` from `load.all_spectra`.

mass_per_bin now takes a normal python callable and a vectorized function as input. 
The latter boosts performance significantly

- CSPEventRate functions are now called:
    - calculate_rate_over_time -> over_time
    - calculate_rate_at_time -> at_time
new functions: 
    - grid_over_time: calculates the rates from a 2D SFH (per BPASS metallicity) from  time_points to time bins
     - grid_at_time: calculate the event rate at a specific moment in lookback time from a 2D SFH grid

* BUG: use np instead of pd in SpectraCompiler.
* BUG: fix index and make test pass.
* BUG: fix another regression bug.
* Fix further regression bug.

Authored-by: Martin Glatzle <mglatzle@mpa-garching.mpg.de>

Also includes some docstring updates

CSPSpectra now has the following functions: 
Public: 
- at_time: at time with function input
- over_time: over lb time with function input
- grid_at_time: at time with SFH grid input
- grid_over_time: over lb time with SFH grid input
Private: 
- grid_rate_calculator_at_time
- grid_rate_calculator_over_time

The grid_rate_calculators_* are numba functions that  have the grid_* around them as wrappers for the input and output

Try/except has been replace by a if/else with a file check.