Improve performance of multivector arithmetic by avoiding copies

[eric-wieser]

Multiplications become roughly 20% faster, and addition 33% faster. There's now a new copy argument to the MultiVector constructor.

Some timings:

Edit: These timings are out of date, and include the effects of #283 which used to be part of this PR. We should retime before merging

After:

In [1]: from clifford import Cl
In [2]: layout, _ = Cl(5)
In [3]: a = layout.randomMV(); b = layout.randomMV()

In [4]: %timeit a * b
The slowest run took 4.76 times longer than the fastest. This could mean that an intermediate result is being cached.
20.4 µs ± 11.3 µs per loop (mean ± std. dev. of 7 runs, 1 loop each)

In [5]: %timeit a * b
8.64 µs ± 341 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

In [6]: %timeit a * b
9.23 µs ± 606 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

In [7]: %timeit a + b
3.9 µs ± 250 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

Before:

In [1]: from clifford import Cl
In [2]: layout, _ = Cl(5)
In [3]: a = layout.randomMV(); b = layout.randomMV()

In [4]: %timeit a * b
The slowest run took 4.37 times longer than the fastest. This could mean that an intermediate result is being cached.
18.6 µs ± 14.2 µs per loop (mean ± std. dev. of 7 runs, 1 loop each)

In [5]: %timeit a * b
11.7 µs ± 759 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

In [6]: %timeit a * b
11.3 µs ± 660 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

In [7]: %timeit a + b
6.11 µs ± 871 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

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[eric-wieser]

I'm wondering if actually we should just change the default to copy=False. I haven't yet found an internal call that would break if we did this.

[lgtm-com[bot]]

This pull request introduces 1 alert and fixes 6 when merging 31ff3173ee700028ea4895ecdeb3a09b1fa8f161 into a5c94317634b048c92fc4f73c738c3c5072c23a2 - view on LGTM.com

new alerts:

• 1 for Syntax error

fixed alerts:

• 3 for Variable defined multiple times
• 2 for Unused import
• 1 for 'import *' may pollute namespace

[eric-wieser]

Let's not merge this till we:

• Decide on #282
• Re-profile it in isolation