Make mepo a module, address functions with module shorthands

[amdasilva]

@tclune @pchakraborty I realize that I am being a little picky here, but these are general suggestions for hepo.

1. Make mepo a bona fide module. Under src/mepo include a proper __init__.py
2. Better yet, rename src/mepo/main.py as__init__.py
3. Because mepo now is a module, from within the module, use python3 style "." for including stuff from within the module. See GMAOpyobs/src/pyobs as an example. Do not have constructs such as:
   from mepo.state.exceptions import SuffixNotRecognizedError
4. Unless you have large submodules, there is no need for excessive subdirectories under src/mepo. If you do need a directory for a submodule, make sure it has its own __init__.py. Otherwise, include the python sources under src/mepo. As a rule of thumb, there should not be a directory without a __init__.py
5. Avoid explicitly importing functions from modules. Instead of Fortran like style (which I used to do)
   from numpy import pi, sin
zero = sin(pi)
   do this
   import numpy as np
zero = np.sin(np.pi)
   This is a basic maintenance device. Otherwise, you have to be editing the import line every time you need another function from numpy. Also, abbreviating packages with 2 letters ("np" for numpy) is an established convention.