:bug::wrench: fix sign error for dtheta_dsigma in NorSand

[jgiven100]

Describe the PR I found a bug related to the dtheta_dsigma sign I had included in the previous NorSand implemention.

Previously we were passing compression positive stresses from NorSand -> dtheta_dsigma(...), BUT dtheta_dsigma(...) returns the same Vector6d regardless of being passed compression positive or tension positive stresses. So this is the incorrect way to account for the incompatibility of sign conventions between NorSand and material utility.

Therefore, we need a -1. coefficient infront of dtheta_dsigma(...) to account for incompatibility of 2 sign conventions

Numerically Approximated Validation Consider following stress state (tension positive):

-1.5
-1.6
-1.7
-1.8
0.0
0.0

Lode angle is computed via material utility (cosine convention): 0.475541833917983

Then change the stress state a bit (tension positive):

-1.5 - 1e-10
-1.6
-1.7
-1.8
0.0
0.0

Lode angle is computed via material utility (cosine convention): 0.47554183393394

Using approximate derivatives for the first term in dtheta_dsigma gives

dtheta_dsigma(0) = (0.47554183393394 - 0.475541833917983) / 1e-10 
dtheta_dsigma(0) = 0.1595701348833245

which approximately matches the first value of the proposed -1. * mpm::materials::dtheta_dsigma(stress) in terms of magnitude and sign

dtheta_disigma(0) : 0.159573
dtheta_disigma(1) : 0.160313
dtheta_disigma(2) : -0.319887
dtheta_disigma(3) : 0.0266367
dtheta_disigma(4) : -0
dtheta_disigma(5) : 0

Subsequent terms can be likewise computed.

Related Issues/PRs N/a

Additional context N/a