Fix install without conda

[evomassiny]

Hi,

First of all, thank you for your work on this package !

I experienced some issues while installing this package in a environment where conda isn't practical.

This PR fixes those issues:

• the setup.py script invoked with the build command did not read the --mkl-root=/path/to/mlk option, which makes the cmake configuration impossible when "mkl" is installed outside of sys.prefix.
• setup.py invoked with install did not reads those options, but expected them to build the cmake arguments, even when "cmake" had already ran.

On a ubuntu 20.04 machine, prepared as follow:

sudo apt-add-repository multiverse # needed for mkl
sudo apt-get update
sudo apt-get install  g++ cmake libeigen3-dev libmkl-dev

git clone https://github.com/CNES/pangeo-pytide.git
cd pangeo-pytide
git submodule update --init --recursive -q

After those small fixes, I was able to install the package by running setup.py directly

python setup.py build --eigen-root /usr/include/eigen3/ --mkl-root /usr/
python setup.py install

(I've also update the associated doc.)

I hope this helps, best regards,

evomassiny

[fbriol]

Thank you very much for your contribution.