make -1 modular symbols from eclib available to elliptic curves

[wuthrich]

This ticket is a follw-up from #10236. In there certain changes from #9476 and #9247 were reverted back. Here is what has to be done to make modular symbols with negative sign available for computations with elliptic curves, e.g. p-adic L-series.

Currently eclib can compute the space of modular symbols of sign = -1, but ECModularSymbol in sage.libs.cremona.newforms.pyx can not use them yet to compute {0,r}-

Once this is there, one can modify sage.schemes.elliptic_curves.ell_modular_symbols.py to allow sign = -1 in ModularSymbolECLIB. (Where one has to think a little bit about the normalization.) This would allow to switch in line 1098 of ell_rational_field.py and in line 131 of padic_lseries.py to make use_eclib=True to be default.

[cremona]

The current eclib (eclib-20100711.spkg) has normalization problems in the "sign=-1" modular symbols, which I am currently working on fixing. The Sage interface should perhaps wait until that is done.