BUG: use np instead of pd in SpectraCompiler.

[findesgh]

This is an alternative to #59, either fixes not being able to load for me. This one uses numpy exclusively pandas.

[maxbriel]

Hi findesgh,

Thank you for the different implementation! The SpectraCompiler issue is solved using the pandas method at the moment. Did you already have a look at the performance this method vs the pandas one?

Since the tests for the SpectraCompiler are a little tricky, I will have a look at them to make them work for this method.

[findesgh]

Yes, I noticed that a few other tests are failing as well. Now I hope that they should pass.

I haven't compared the performance, no. I just think using numpy is a bit easier in this case. Just a suggestion, though.

[maxbriel]

The numpy method looks cleaner and less bloated, so I agree! With the updates test, I have merged it with the csp branch.
Thank you for the contribution!