1. Method for obtaining an isolated extract of the wild plant Cyclamen europaeum L., which is characterised in that an aqueous extraction is carried out which comprises: a) a first stage of pressing of the previously cleaned and prepared tuber; a-i) preparing the solid fraction, residues from the pressing, by means of addition of water in a ratio of 1: 0.5-1.8 a-ii) thermostat-controlling the prepared mass to a temperature of between 20 degree C and 80 degree C; and a-iii) finally, in a mixing reactor, mixing the liquid fraction from the second pressing of extract of residues with the liquid fraction from the first pressing. b) a second stage of purification of the liquid fraction obtained following the pressing, which includes: b-i) the addition of an alcohol to said liquid fraction; b-ii) its subsequent storage in a refrigeration chamber; b-iii) finally, filtering. 2. Method as claimed in Claim 1, characterised in that stage b-ii) is carried out at a temperature between 2 degree C and 25 degree C, preferably between 4 degree C and 8 degree C, for 10-48 hours, preferably for 18-20 hours. 3. Method as claimed in Claim 1, characterised in that stage b-iii) of filtering is carried out in two steps: first through a primary purification filter, and then through a fine-pore bactericidal filter, at a pressure of 0.25+-0.005 MPa. 4. Method as claimed in Claim 1, characterised in that in stages: a-i) the solid fraction is prepared by means of addition of water in a ratio of 1: 0.64-0.66; and a-ii) the prepared mass is thermostat-controlled to a temperature between60 C and70 C for 1 hour, and a second pressing is then carried out. 5. Method for obtaining an isolated extract of the wild plant Cyclamen europaeum L., which is characterised in that an alcoholic extraction is carried out which comprises: a) a first stage of trituration of the previously cleaned and prepared tuber; b) a second stage of extraction of the biologically active substances, which comprises: b-i) the addition of an alcohol to the triturate of tubers in a ratio by volume of tuber/solvent of between 1: 1 and 1: 5; b-ii) the prepared mass is brought to boiling point and left to rest before the alcoholic extraction is carried out; c) a third stage of purification of the alcoholic extraction, which comprises: c-i) the addition of a sorbent in a ratio of mass of raw material to mass of sorbent of between 30: 0.5 and 30: 2; c-ii) purification in refrigerant under reflux; and c-iii) finally, filtering. 6. Method as claimed in Claim 5, characterised in that in stages: b-1) the addition of an alcohol to the triturate of tubers in a ratio by volume of tuber/solvent of 1: 3; b-ii) the prepared mass is brought to boiling point for 1 hour before the alcoholic extraction is carried out; and in stage: c-i) the addition of a sorbent is carried out in a ratio of mass of raw material to mass of sorbent of between 30: 1. 7. Method as claimed in Claim 5, characterised in that a second extraction b-iii) is carried out under the same conditions as the extraction of stages b-i) and b-ii), and mixing of both liquid fractions in the mixer. 8. Method as claimed in either of Claims 5 or 6, characterised in that the sorbent is selected from among activated carbon, silica gel, clay and grade II aluminium oxide, being preferably grade II aluminium oxide. 9. Utilisation of an isolated extract of the wild plant Cyclamen europaeum L., as claimed in any of the above claims, for the manufacturing of a medicament for treating sinusitis.