Fix PISCES test

[nanophyto]

The PISCES test is currently commented out and is thus not running during CI.

When running locally the mass conservation also fails:

[ Info: Testing PISCES element conservation (C, Fe, P, Si, N)
PISCES: Test Failed at /home/phyto-2/OceanBioME.jl-1/test/test_PISCES.jl:89
  Expression: isapprox(total_silicon_tendencies, 0, atol = 10 ^ -30)
   Evaluated: isapprox(3.308722450212111e-24, 0; atol = 1.0e-30)

Stacktrace:
 [1] macro expansion
   @ ~/.julia/juliaup/julia-1.10.10+0.x64.linux.gnu/share/julia/stdlib/v1.10/Test/src/Test.jl:672 [inlined]
 [2] test_PISCES_conservation()
   @ Main ~/OceanBioME.jl-1/test/test_PISCES.jl:89
 [3] macro expansion
   @ ~/OceanBioME.jl-1/test/test_PISCES.jl:174 [inlined]
 [4] macro expansion
   @ ~/.julia/juliaup/julia-1.10.10+0.x64.linux.gnu/share/julia/stdlib/v1.10/Test/src/Test.jl:1577 [inlined]
 [5] top-level scope
   @ ~/OceanBioME.jl-1/test/test_PISCES.jl:173

I have uncommented the PISCES test and increased the Si tolerance to match the nitrogen tendency.

closes #344

[jagoosw]

Thank you for catching!