Add more classes / functions to the top-level __init__.py

[hombit]

Feature request

It would be really helpful to add more stuff to the top-level __init__.py to make it easier to from hipscat import XXX and hipscat.XXX.

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• I have described the purpose of the suggested change, specifying what I need the enhancement to accomplish, i.e. what problem it solves.
• I have included any relevant links, screenshots, environment information, and data relevant to implementing the requested feature, as well as pseudocode for how I want to access the new functionality.
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[nevencaplar]

@hombit Can you please wrote just a couple of example of what imports you would like to have avaliable in this fashion.

[hombit]

@nevencaplar the whole user-level interface with non-confusing names. I think it will be easier to write a pull-request than listing here. In my code I frequently use stuff from pixel_math