Mtf2json

* Unified `tech_base` strings for all components (and mechs). Available strings are:
* `IS`
* `Clan`
* `All`
* `Mixed`
* `Unknown`
* Renamed the mech `techbase` to `tech_base`
* this makes it consistent with other components
* The following components now have a tech base entry:
* Engine
* Armor
* Structure
* Mech
* Added new `engine` section to the JSON structure. Example:

"engine": {
"name": "380 XL Engine",
"type": "XL",
"tech_base": "IS"
},


**Known issues:**
* Some MTF files contain components with two different tech bases
* e.g. the `Turkina U` has the following engine: `engine:285 XL (Clan) Engine(IS)`
* in this case, `mtf2json` will either choose the tech base that corresponds to the mech tech base, or the first one (if the mech has `Mixed` tech base)
* Some MTF files contain conflicting tech bases
* e.g. the `Turkina U` has tech base `Clan` but an `IS` structure (according to the MTF file)
* no workaround has been implemented for this (yet)
* Both issues are very likely MegaMek bugs

* Added support for legacy-style sizeable equipment (e.g. `(5 tons)` or `(1 ton)` substrings instead of `:SIZE:`)
* they are also added to the `equipment` section (see https://github.com/juk0de/mtf2json/releases/tag/v0.2.5)

* Format change: renamed `weapon` to `name` in weapon entries
* New JSON section: `equipment`
* Items with a `:SIZE:` value are now correctly supported and stored in the new `equipment` section.
* E. g. `Liquid Storage (OMNIPOD):SIZE:1.0 (ARMORED)` will be stored as:

"equipment": [
{
"name": "Cargo (Liquid)",
"location": "left_torso",
"type": "transport bay",
"tags": [
"armored",
"omnipod"
],
"size": "1t"
},
]

* More items will be stored in the `equipment` section in future releases

- New handled keys:
- `base_chassis_heat_sinks`
- `ejection`
- `notes`
- The following keys are now explicitly ignored:
- `fluffimage`
- `imagefile`
- `nocrit`
- Added new option `--statistics`
- Refactoring and test improvements

- The `weapons` section is now a list of dictionaries
- this breaks compatibility with the old format
- Weapon quirks are now stored in the `weapon_quirks` section
- before that, only the last `weaponquirk` key was stored (verbatim) in case there were multiple of them
- The `mtf2json` version is now stored as the first key in every JSON file

- Empty values are now allowed (fixes errors for MTF files with e. g. empty `model` entry)
- Added `clanname` to the list of known keys
- Test improvements