Add the function to update the pressure gradient in the hybrid solver

[frankfeifan]

In this PR, we have added a function in SinglePhaseHybridFVM to compute the cell-wise pressure gradient. Basically, the pressure gradient in a cell is computed according to the pressures on the cell center and all faces using a least squares method. So a routine to compute the least squares solution of a linear system is also added.

[paveltomin]

@frankfeifan move into merge queue?

[frankfeifan]

@frankfeifan move into merge queue?

I will have to merge the develop and rebaseline the tests, and then we can move it to merge queue.

[paveltomin]

@frankfeifan move into merge queue?

I will have to merge the develop and rebaseline the tests, and then we can move it to merge queue.

thanks, merge to develop is one button on github click, right? rebaseline does not make much sense before PR is on top of the queue (@CusiniM please correct me if i am wrong about that)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 51.00000% with 49 lines in your changes are missing coverage. Please review.

Project coverage is 53.57%. Comparing base (44684e1) to head (0c05252).

Files	Patch %	Lines
...sSolvers/fluidFlow/SinglePhaseHybridFVMKernels.hpp	0.00%	39 Missing :warning:
.../physicsSolvers/fluidFlow/SinglePhaseHybridFVM.cpp	0.00%	8 Missing :warning:
...nents/physicsSolvers/fluidFlow/SinglePhaseBase.hpp	0.00%	2 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2110      +/-   ##
===========================================
- Coverage    53.58%   53.57%   -0.01%     
===========================================
  Files         1003     1003              
  Lines        85189    85289     +100     
===========================================
+ Hits         45645    45694      +49     
- Misses       39544    39595      +51     

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