Kalman performance tweaks

[jlperla]

This prepares things to have a preallocated cache and implements things inplace where possible.

[coveralls]

Pull Request Test Coverage Report for Build 1854604071

• 136 of 137 (99.27%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+3.07%) to 84.097%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/kalman.jl	136	137	99.27%

Totals
Change from base Build 1842991448:	3.07%
Covered Lines:	312
Relevant Lines:	371

---

💛 - Coveralls

[codecov]

Codecov Report

Merging #43 (7b662fb) into main (b06d704) will increase coverage by 3.06%.
The diff coverage is 99.27%.

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
+ Coverage   81.02%   84.09%   +3.06%     
==========================================
  Files           8        8              
  Lines         311      371      +60     
==========================================
+ Hits          252      312      +60     
  Misses         59       59              

Impacted Files	Coverage Δ
src/DifferenceEquations.jl	25.00% <ø> (ø)
src/kalman.jl	99.34% <99.27%> (+0.42%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b06d704...7b662fb. Read the comment docs.