mrc-6176 run discrete system

[EmmaLRussell]

This branch builds on the original WIP commit for a discrete system, which applies a generator to a state. Various bits of tidying up, validation and tests.

A couple of things to note:

• I've added SystemState to represent the multidimensional state array, whose values can be fetched and updated per particle. This is similar to the implementation in original dust-js, and uses the scijs ndarray rather than the stdlib ndarry, which turns out to be unwritable by slice! However, the Packer still uses stdlib's ndarray! I'll fix this in another ticket.
• Rather than having the DiscreteSystem taking the generator's generic types TShared and TInternal (which seemed weird). it's now taking the generator type TGenerator instead, and using infer to work out the correct Shared and Internal types from whatever Generator gets fed in. The syntax for this is weird, and is oddly tied to conditional types. @M-Kusumgar may have some thoughts on this!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (789e29e) to head (3b5c9c1).

Additional details and impacted files

@@            Coverage Diff            @@
##              main        #5   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            3         5    +2     
  Lines           60       139   +79     
  Branches        11        15    +4     
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+ Hits            60       139   +79     

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