allow to use read.snapshot without filtering, and to pass filter.function

[orichters]

• if you don't supply a filter but just want to profit from the correct quote and na settings, don't fail if grep, head and tail are not available, but just read it
• allow to pass a filter.function to read.quitte, and transform the list-based filtering into filter function
• I took out the temperature variable, because somehow read.quitte on Windows transforms °C from the mocked IIASA snapshot mif file written with write.table into <U+00B0>C and I don't want to debug that… Writing with write.mif is fine, so no need to worry.