Support using not with a context manager

[kylebarron]

Currently there's a significant amount of initialization done in __enter__ instead of __init__. This means that code will silently fail if not used with a context manager:

scene = S2COGReader('S2B_12SYJ_20200901_0_L2A')
# no error
scene.center
# errors:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-77-5a7975690aa4> in <module>
----> 1 self.center

<ipython-input-43-587f5a19de47> in center(self)
     41         """Dataset center + minzoom."""
     42         return (
---> 43             (self.bounds[0] + self.bounds[2]) / 2,
     44             (self.bounds[1] + self.bounds[3]) / 2,
     45             self.minzoom,

AttributeError: 'S2COGReader' object has no attribute 'bounds'

I believe __init__ should be used for any necessary initialization of the class. __enter__ should only instantiate context-specific attributes like a file descriptor or a database cursor. Such a context really doesn't apply here, and __enter__ is essentially on there for syntactical sugar. Hence initialization should be move entirely to __init__.

[vincentsarago]

We built rio_tiler.io.BaseReader with only __enter__ and __exit__ to force the user to use context manager. I understand there is no need for CTX in rio-tiler-pds, mostly because we don't need to close or delete any resource. but I'd like to keep this syntax. I'm ok for moving stuff outisde the __enter__ but I'll keep the documentation as it is.

Note: because we are using attrs we don't need __init__ but we will need instead a __attrs_post_init__ method.

[vincentsarago]

@kylebarron We can move forward once cogeotiff/rio-tiler#271 will be merged ;-)