Clarify expression alias logic

[evanpurkhiser]

Instead of using tuple indexing use a namedtuple mapped from the expression aliases. This makes readability / understanding of this logic a little easier.

[evanpurkhiser]

Also pulls this up into the module scope so we're not redeclaring this mapping every time

[kiorky]

moving the aliases top module level would be ok, but not in favor of a namedtuple because users shall be allowed to update easily the aliases.

Another problem is that this change is not at all retrocompatible. To allow retrocompat, maybe using a class attribute that dictcopy the module level one would be a solution.

[kiorky]

Damn, the above comment was not validated, so you didnt saw it... sorry :(

[evanpurkhiser]

users shall be allowed to update easily the aliases.

What makes it difficult to update the aliases because we're using a named tuple? Could you share an example of what you're thinking that is more difficult?

[kiorky]

dict are mutable, not namedtuple.

[evanpurkhiser]

You could just replace it like this though right

# Replace alias with whatever you want
EXPR_ALIASES['@midnight'] = ExpressionAlias("1 2 * * *", "h h * * *")

[evanpurkhiser]

Updated to fix remove the conflict

[kiorky]

Updated to fix remove the conflict

oups, the other pr i just merged broke it again :( :) .

[evanpurkhiser]

I'll get it fixed after I get the black formatting done :)