so that we use the exact same file as in evalhyd-python/evalhyd-r

resolves #10

fixes #5

fixes #9

Closes #7, #8, and #6

See merge request !6

resolves #8

resolves #7

related to evalhyd-r#6

for arrays, rank is not applicable, so it should not be checked

it had no effect on numerical results

`xt::median` and `xt::quantile` are not NaN-proof, and do not have
equivalent NaN functions (like `xt::mean` and `xt::nanmean`) so
it required a filter to drop the NaN before computing the corresponding
condition value

e.g. "q_prd_median{>quantile0.3,<quantile0.7}" was not working
because the string was too long, but the regex were also quite
complicated, so this revises the approach and simplifies the regex

but to overcome the length limitation with C++ regex, "quantile" now
need to be specified as "qtl" to be shorter

"evalhyd-cpp" is both the core library to use in bindings, and a
C++ header-only library in its own write that can be used by C++ users

the energy score (ES) is a multi-variate (multi-site)
generalisation of the CRPS

new metric is called CRPS_FROM_ECDF

new metric is called CRPS_FROM_BS, and existing metric is renamed
CRPS_FROM_QS

available in xtensor 0.24., see https://github.com/xtensor-stack/xtensor/issues/2651