New FsGrid operators and lerp method

[kstppd]

-This PR implements new operators for FsGrid .
-Now one can use +, -,+=,-=,*,*=,/,/= with FsGrid data structures.
-There is also a new lerp_t method which linearly interpolates between two existing FsGrids. It can be used like this:
-This PR also updates the tests so handles #20 and that can be closed if you want.

  FsGrid<std::array<Real, 10>, 2> res=lerp_t(A,B,t_a,t_b,t);

where

• A,B are FsGrids of same signature
• t_a,t_b are timestamps that corespond to A and B
• t is the timestamp of the interpolated FsGrid res

I have implemented a few ways to make it more safe. For example we do not want to add a string to a perB field I guess :D
I also have a new test test/operators.cpp that demos some of the added functionality.
I will be doing more tests to catch any bugs or edge cases. So this is WIP.
Let me know what you think and what I should change :)

[ursg]

This looks pretty nice! Also, this is a wonderful step in the direction of a fully vectorized fieldsolver implmentation (although I dread the day that anyone of us would have to reformulate the hall component expressions into a new syntax).

Are there any simple parts of the fieldsolver where this could be directly taken into use, just to compare the performance?

[markusbattarbee]

Appears to run, and the interpolation at least doesn't break testpackage runs completely :)
However, note that the tests/operators.cpp interpolation test still uses the old interface of three time values.

[kstppd]

@markusbattarbee Right Markus I forgot to push the changes to that so it does not even compile now. Will be fixed in the next push that has a few more operators coming