Trade analyzer

## Motivation ##
More equipment equals more upgrade equals more fun(?).

This makes the recently added "econ improv" to Economy Info (#3081) depend on the player having bought a "Trade Analyzer" module: 0t, $400, and has its own slot.

In my opinion, there shouldn't be too much automation when starting the game, it should be something you only buy once you know your way around, to cut the corners of tedious work, so to speak.

## Two commits ##
The first commit is just a suggested unrelated cleanup. Take it or leave it. Second commit adds the trade analyzer. They're independent of each other.

## Known Bug ##
This is WIP since there is a small bug, that I haven't had time to investigate, but it's just that the econ trade screen is not updated immediately when the player has bought/sold the trade analyzer, thus the player needs to go to system view and (re)select a star system for it to kick in. I guess it's a single liner that should go somewhere.

I'm submitting this PR now, and see if anyone know right away where the update should be made.

## Neat? ##
Issue number = starting year
Please don't use C-style casts. It causes warnings in *pedantic* mode. I think in this case, you can just omit the cast (or use functional notation ```bool(trade_analyzer)```)