Add sum of expectation

[Katalam]

When merge this will:

• add .idea folder to git ignore
• add a strict check to prime expectation
• add a sum of expectation

[faissaloux]

Hello @Katalam

When I'll use toBeSumOf(), I'll expect to sum up the first argument like the other methods toBePowerOf() and toBeFactorialOf() ...

So to be consistent and not confusing, can you move the callable $step to the first position and make it able to accept an array as well, so it either sum up the array of numbers or the callable with $from to $to (Rename those params too please).

Thank you

[faissaloux]

I'm thinking about separate this to two methods:

• toBeSumOf() should accept only an array of numbers.
• Another one that accepts $step, $from and $to (This order should be respected) .

[faissaloux]

ping @Katalam

[Katalam]

I broke my forked git tree somehow, but I removed all the unwanted commit changes

[faissaloux]

@Katalam thanks for your contribution!